Document and Entity Information - USD ($)	12 Months Ended
Document and Entity Information - USD ($)	Dec. 31, 2017	Mar. 15, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	FY
Trading Symbol	LIFE
Entity Registrant Name	aTYR PHARMA INC
Entity Central Index Key	0001339970
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Smaller Reporting Company
Entity Common Stock, Shares Outstanding		29,797,663
Entity Public Float			$ 45,336,025

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 21,091	$ 38,388
Available-for-sale investments, short-term	64,028	33,759
Prepaid expenses and other assets	1,866	2,621
Total current assets	86,985	74,768
Available-for-sale investments, long-term		4,002
Property and equipment, net	2,280	1,421
Other assets	90	333
Total assets	89,355	80,524
Current liabilities:
Accounts payable	2,276	2,606
Accrued expenses	3,103	5,450
Current portion of deferred rent		130
Current portion of long-term debt, net of issuance costs and discount	5,012	339
Total current liabilities	10,391	8,525
Long-term debt, net of current portion and issuance costs and discount	14,719	9,198
Commitments and contingencies (Note 5)
Stockholders’ equity:
Preferred stock, $0.001 par value; undesignated authorized shares – 5,000,000 at December 31, 2017 and 2016, respectively; Class X Convertible Preferred Stock issued and outstanding shares– 2,285,952 and no shares at December 31, 2017 and 2016, respectively	2
Common stock, $0.001 par value; authorized shares – 150,000,000 as of December 31, 2017 and 2016, respectively; issued and outstanding shares – 29,789,162 and 23,744,832 as of December 31, 2017 and 2016, respectively	30	24
Additional paid-in capital	328,519	278,832
Accumulated other comprehensive loss	(120)	(76)
Accumulated deficit	(264,186)	(215,979)
Total stockholders’ equity	64,245	62,801
Total liabilities and stockholders’ equity	$ 89,355	$ 80,524

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	29,789,162	23,744,832
Common stock, shares outstanding	29,789,162	23,744,832
Class X Convertible Preferred Stock [Member]
Preferred stock, shares issued	2,285,952	0
Preferred stock, shares outstanding	2,285,952	0

Consolidated Statements of Operations - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating expenses:
Research and development	$ 30,067	$ 42,846	$ 34,504
General and administrative	17,078	15,094	13,112
Total operating expenses	47,145	57,940	47,616
Loss from operations	(47,145)	(57,940)	(47,616)
Other income (expense), net
Other income (expense), net	(1,062)	65	(386)
Loss on extinguishment of debt		(29)
Change in fair value of warrant liabilities			29
Total other income (expense)	(1,062)	36	(357)
Loss before income taxes	(48,207)	(57,904)	(47,973)
Income tax benefit		49
Net loss	(48,207)	(57,855)	(47,973)
Accretion to redemption value of redeemable convertible preferred stock			(15)
Net loss per share attributable to common stockholders, basic and diluted	$ (48,207)	$ (57,855)	$ (47,988)
Net loss per share, basic and diluted	$ (1.87)	$ (2.44)	$ (3.03)
Weighted average common shares outstanding, basic and diluted	25,799,853	23,681,019	15,838,353

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (48,207)	$ (57,855)	$ (47,973)
Other comprehensive gain (loss):
Change in unrealized gain (loss) on available for sale investments, net of tax	(44)	95	(171)
Comprehensive loss	$ (48,251)	$ (57,760)	$ (48,144)

Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Stockholder Note Receivable [Member]	Other Comprehensive Loss [Member]	Accumulated Deficit [Member]	Redeemable Convertible Preferred Stock [Member]	Convertible Preferred Stock [Member]
Redeemable Convertible Preferred Stock, Beginning balance at Dec. 31, 2014							$ 95,619
Redeemable Convertible Preferred Stock, shares, Beginning balance at Dec. 31, 2014							73,487,415
Beginning balance at Dec. 31, 2014	$ (91,010)	$ 1	$ 19,209	$ (69)		$ (110,151)
Beginning balance, Shares at Dec. 31, 2014		909,880
Issuance of Series E redeemable convertible preferred stock for cash							$ 75,650
Issuance of Series E redeemable convertible preferred stock for cash, Shares							68,166,894
Conversion of redeemable convertible preferred stock in connection with initial public offering	171,284	$ 16	171,268				$ (171,284)
Conversion of redeemable convertible preferred stock in connection with IPO, Shares		16,279,859					(141,654,309)
Issuance of common stock through initial public offering, net	75,903	$ 6	75,897
Issuance of common stock through initial public offering, net, Shares		6,164,000
Repayment of stockholder note receivable	60		(9)	$ 69
Exercise of common stock options	535	$ 1	534
Exercise of common stock options, Shares		196,500
Reclassification of preferred stock warrant liability to additional paid-in-capital	290		290
Issuance of common stock to The Scripps Research Institute	1,411		1,411
Issuance of common stock to The Scripps Research Institute, Shares		119,840
Changes in share repurchase liability	(120)		(120)
Stock-based compensation	4,856		4,856
Accretion to redemption value of redeemable convertible preferred stock	(15)		(15)				$ 15
Net unrealized gain (loss) on investments, net of tax	(171)				$ (171)
Net loss	(47,973)					(47,973)
Ending balance at Dec. 31, 2015	115,050	$ 24	273,321		(171)	(158,124)
Ending balance, Shares at Dec. 31, 2015		23,670,079
Exercise of common stock options and release of restricted stock units	20		20
Exercise of common stock options and release of restricted stock units, Shares		17,972
Issuance of common stock pursuant to employee stock purchase plan	143		143
Issuance of common stock pursuant to employee stock purchase plan, Shares		56,781
Issuance of warrants related to term loan	217		217
Changes in share repurchase liability	102		102
Stock-based compensation	5,029		5,029
Net unrealized gain (loss) on investments, net of tax	95				95
Net loss	(57,855)					(57,855)
Ending balance at Dec. 31, 2016	62,801	$ 24	278,832		(76)	(215,979)
Ending balance, Shares at Dec. 31, 2016		23,744,832
Exercise of common stock options and release of restricted stock units	$ 186		186
Exercise of common stock options and release of restricted stock units, Shares		111,039
Exercise of common stock options, Shares	75,289
Issuance of common stock pursuant to employee stock purchase plan	$ 175		175
Issuance of common stock pursuant to employee stock purchase plan, Shares		61,171
Issuance of common stock and preferred stock from private placement, net of offering costs	42,239	$ 6	42,231					$ 2
Issuance of common stock and preferred stock from private placement, net of offering costs, Shares		5,872,120						2,285,952
Issuance of warrants related to term loan	263		263
Changes in share repurchase liability	48		48
Stock-based compensation	6,784		6,784
Net unrealized gain (loss) on investments, net of tax	(44)				(44)
Net loss	(48,207)					(48,207)
Ending balance at Dec. 31, 2017	$ 64,245	$ 30	$ 328,519		$ (120)	$ (264,186)		$ 2
Ending balance, Shares at Dec. 31, 2017		29,789,162						2,285,952

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net loss	$ (48,207)	$ (57,855)	$ (47,973)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	713	900	869
Issuance of common stock for technology			1,411
Stock-based compensation	6,784	5,029	4,856
Accretion of debt discount	192	173	297
Loss on debt extinguishment		29
Change in fair value of preferred stock warrant liability			(29)
Amortization of premium of available-for-sale investment securities	14	531	789
Deferred rent	(130)	(315)	(295)
Changes in operating assets and liabilities
Prepaid expenses and other assets	761	(421)	(666)
Accounts payable and accrued expenses	(2,491)	(932)	3,944
Net cash used in operating activities	(42,364)	(52,861)	(36,797)
Cash flows from investing activities:
Purchases of property and equipment	(1,312)	(600)	(664)
Purchases of available-for-sale investment securities	(77,672)	(28,089)	(109,445)
Maturities of available-for-sale investment securities	51,347	62,216	38,115
Net cash (used in) provided by investing activities	(27,637)	33,527	(71,994)
Cash flows from financing activities:
Proceeds from issuance of preferred stock for cash, net of issuance costs			75,648
Proceeds from issuance of common stock through initial public offering, net of offering costs			76,902
Proceeds from issuance of common stock through option exercises	186	20	604
Proceeds through employee stock purchase plan	175	143
Proceeds from borrowing, net	9,866	9,736
Proceeds from issuance of securities through private placement, net of issuance costs	42,477
Repayments on loans payable		(5,202)	(3,237)
Repayments of convertible debt			(2,000)
Net cash provided by financing activities	52,704	4,697	147,917
Net change in cash and cash equivalents	(17,297)	(14,637)	39,126
Cash and cash equivalents at beginning of period	38,388	53,025	13,899
Cash and cash equivalents at the end of period	21,091	38,388	53,025
Supplemental disclosure of cash flow information:
Interest paid	1,000	225	925
Supplemental schedule of noncash investing and financing activities:
Issuance of warrants in connection with borrowings	263	217
Changes in share repurchase liability	$ 48	$ 102	$ (120)

Organization, Business and Basis of Presentation	12 Months Ended
Organization, Business and Basis of Presentation	Dec. 31, 2017
Accounting Policies [Abstract]
Organization, Business and Basis of Presentation	1. Organization, Business and Basis of Presentation Organization and Business We were incorporated in the state of Delaware on September 8, 2005. We are focused on the discovery and clinical development of innovative medicines using our knowledge of tRNA synthetase biology. Principles of Consolidation Our consolidated financial statements include our accounts, our 98% majority-owned subsidiary in Hong Kong, Pangu BioPharma Limited (Pangu BioPharma). All intercompany transactions and balances are eliminated in consolidation. Use of Estimates Our consolidated financial statements are prepared in accordance with generally accepted accounting principles (GAAP). The preparation of our consolidated financial statements requires us to make estimates and assumptions that impact the reported amounts of assets, liabilities and expenses and the disclosure of contingent assets and liabilities in our consolidated financial statements and accompanying notes. The most significant estimates in our consolidated financial statements relate to the fair value of equity issuances and awards, and clinical trials and research and development expenses. Although these estimates are based on our knowledge of current events and actions we may undertake in the future, actual results may ultimately differ materially from these estimates and assumptions. Segment Reporting Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. We view our operations and manage our business in one operating segment.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Cash and Cash Equivalents

Cash and cash equivalents consist primarily of readily available checking, money market accounts and money market funds. We consider all highly liquid investments that mature in three months or less when purchased to be cash equivalents.

Investment Securities

Investment securities primarily consist of investment grade corporate debt securities, asset-backed securities, commercial paper and United States Treasury securities. We classify all investment securities as available-for-sale. Investment securities are carried at fair value, with the unrealized gains and losses, if any, reported as a component of other comprehensive income (loss) in stockholders’ equity (deficit) until realized. Realized gains and losses from the sale of investment securities, if any, are determined on a specific identification basis. A decline in the market value of any investment security below cost that is determined to be other than temporary will result in an impairment charge to earnings and a new cost basis for the security is established. No such impairment charges were recorded for any period presented. Premiums and discounts are amortized or accreted over the life of the related security as an adjustment to yield using the straight-line method and are included in interest income. Interest income is recognized when earned. As of December 31, 2017, we held an aggregate total of $64.0 million of investment securities which consisted of corporate debt securities, asset-backed securities, all of which will mature in less than one year and there was a $0.1 million difference between the amortized cost and fair value of these investment securities. As of December 31, 2016, we held $37.8 million of corporate debt securities, all of which mature in less than one year, and there was $27,000 difference between the amortized cost and fair value of these investment securities.

Concentration of Credit Risk

Financial instruments that potentially subject us to significant concentration of credit risk consist primarily of cash, cash equivalents and investment securities. We have established guidelines regarding diversification of investments and their maturities, which are designed to maintain principal and maximize liquidity. We maintain deposits in federally insured financial institutions in excess of federally insured limits. We have not experienced any losses in such accounts and we believe that we are not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held.

Property and Equipment

Property and equipment are stated at cost and depreciated on a straight-line basis over the estimated useful life of the related assets (generally three to seven years). Leasehold improvements are stated at cost and amortized on a straight-line basis over the lesser of the remaining term of the related lease or the estimated useful life of the leasehold improvements. Repairs and maintenance costs are charged to expense as incurred.

Impairment of Long-Lived Assets

Long-lived assets consist primarily of property and equipment. An impairment loss is recorded if and when events and circumstances indicate that assets might be impaired and the undiscounted cash flows estimated to be generated by those assets are less than the carrying amount of those assets. While our current and historical operating losses are indicators of impairment, we believe that future cash flows to be received support the carrying value of our long-lived assets and, accordingly, have not recognized any impairment losses since inception.

Accrued Expenses

As part of the process of preparing our consolidated financial statements, we are required to estimate accrued expenses, including accrued research and development expenses for fees paid to investigative sites and clinical research organizations (CROs) in connection with clinical trials; service providers in connection with preclinical development activities; service providers related to product manufacturing; and other professional services. The accrual process involves reviewing open contracts and purchase orders, communicating with our personnel to identify services that have been performed on our behalf and estimating the level of service performed and the associated cost incurred for the service when we have not yet been invoiced or otherwise notified of the actual cost. We make estimates of accrued expenses as of each balance sheet date in our consolidated financial statements based on facts and circumstances known to us at that time. Although we do not expect the estimates to be materially different from amounts actually incurred, if the estimates of the status and timing of services performed differs from the actual status and timing of services performed, we may report amounts that are too high or too low in any particular period. Historically, our estimated accrued liabilities have approximated actual expenses incurred. Subsequent changes in estimates may result in a material change in our accruals.

Research and Development Costs

Research and development costs are expensed as incurred. Research and development costs include: salaries and employee-related expenses, including stock-based compensation and benefits for personnel in research and product development functions; costs associated with conducting our preclinical, development and regulatory activities, including fees paid to third-party professional consultants, service providers and our scientific, therapeutic and clinical advisory boards; costs to acquire, develop and manufacture preclinical study and clinical trial materials; costs incurred under clinical trial agreements with clinical research organizations and investigative sites; costs for laboratory supplies; payments related to licensed products and technologies; allocated facilities and information technology costs; and depreciation.

Patent Costs

Costs related to filing and pursuing patent applications are recorded as general and administrative expense and expensed as incurred since recoverability of such expenditures is uncertain.

Stock-Based Compensation

Stock-based compensation expense represents the cost of the grant date fair value of employee stock option and restricted stock unit grants recognized over the requisite service period of the awards (usually the vesting period) on a straight-line basis. We recognize forfeitures as they occur as a reduction of expense. For stock option grants with performance-based milestones, the expense is recorded over the service period after the achievement of the milestone is probable or the performance condition is achieved. For stock option grants with market-based conditions, the expense is recorded using the accelerated attribution method over the requisite service period for each vesting tranche. We account for stock options granted to non-employees using the fair value approach. These option grants are subject to periodic revaluation over their vesting terms. We estimate the fair value of employee and non-employee stock option grants using the Black-Scholes option pricing model. We estimate the fair value of the market-based stock option grants using a Monte Carlo simulation. The fair value of restricted stock units is determined by the closing price as of the grant date.

Income Taxes

We account for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements. Under this method, deferred tax assets and liabilities are determined on the basis of the differences between the financial statements and tax basis of assets and liabilities using enacted tax rates in effect for the year in which the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized as income in the period that includes the enactment date.

We recognize net deferred tax assets to the extent that we believe these assets are more likely than not to be realized. In making such a determination, we consider all available positive and negative evidence, including future reversals of existing taxable temporary differences, projected future taxable income, tax-planning strategies and results of recent operations. If we determine that we would be able to realize the deferred tax assets in the future in excess of their net recorded amount, we would make an adjustment to the deferred tax asset valuation allowance, which would reduce the provision for income taxes.

We record uncertain tax positions on the basis of a two-step process whereby (1) we determine whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (2) for those tax positions that meet the more-likely-than-not recognition threshold, we recognize the largest amount of tax benefit that is more than 50% likely to be realized upon ultimate settlement with the related tax authority. We recognize interest and penalties related to unrecognized tax benefits within income tax expense. Any accrued interest and penalties are included within the related tax liability.

In March 2016, the FASB issued ASU No. 2016-09, Compensation-Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting, which involves several aspects of the accounting for share-based payment transactions, including income tax consequences, classification of awards as either equity or liabilities, and classification on the statement of cash flows. ASU 2016-09 will be effective for the annual periods beginning after December 15, 2016 and interim periods within those annual periods, with early adoption permitted. We elected to early adopt this guidance beginning in the year ended December 31, 2016. Upon adoption, the balance of the unrecognized excess tax benefits will be reversed with the impact recorded to retained earnings net of any change to the valuation allowance as a result of the adoption. Due to the full valuation allowance on the U.S. deferred tax assets, there was no material impact in our consolidated financial position or results of operations upon adoption.

Net Loss Per Share

Basic net loss per share is calculated by dividing the net loss attributable to common stockholders by the weighted average number of common shares outstanding for the period, without consideration for common stock equivalents and adjusted for the weighted average number of common shares outstanding that are subject to repurchase. We have excluded 3,685, 25,984 and 61,814 shares subject to repurchase from the weighted average number of common shares outstanding for the years ended December 31, 2017, 2016 and 2015, respectively. Diluted net loss per share attributable to common stockholders is calculated by dividing the net loss attributable to common stockholders by the weighted average number of common stock equivalents outstanding for the period determined using the treasury-stock method. Dilutive common stock equivalents are comprised of convertible preferred stock, warrants for common stock, options and restricted stock units outstanding under our stock option plan and estimated shares to be purchased under our employee stock purchase plan. For all periods presented, there is no difference in the number of shares used to calculate basic and diluted shares outstanding due to our net loss position.

Potentially dilutive securities not included in the calculation of diluted net loss per share because to do so would be anti-dilutive are as follows (in common share equivalents):

	Year Ended December 31,
	2017		2016		2015
Class X Convertible Preferred Stock (if-converted)		11,429,760		—		—
Warrants for common stock		6,682,708		121,512		25,970
Common stock options and restricted stock units		4,666,359		4,091,701		2,625,280
Employee stock purchase plan		31,086		36,836		17,363
		22,809,913		4,250,049		2,668,613

The following table summarizes our net loss per share (in thousands, except per share data):

	Year Ended December 31,
	2017			2016			2015
Numerator:
Consolidated net loss	$	(48,207	)	$	(57,855	)	$	(47,973	)
Accretion to redemption value		—			—			(15	)
Net loss attributable to common stockholders		(48,207	)		(57,855	)		(47,988	)

Denominator:
Weighted average common shares outstanding		25,803,538			23,707,003			15,900,167
Weighted average common shares subject to repurchase		(3,685	)		(25,984	)		(61,814	)
Weighted average common shares outstanding - basic and diluted		25,799,853			23,681,019			15,838,353

Net loss per share - basic and diluted	$	(1.87	)	$	(2.44	)	$	(3.03	)

Convertible Preferred Stock

We apply the relevant accounting standards to distinguish liabilities from equity when assessing the classification and measurement of preferred stock. Preferred shares subject to mandatory redemptions are considered liabilities and measured at fair value. Conditionally redeemable preferred shares are considered temporary equity. All other preferred shares are considered as stockholders’ equity. None of our outstanding preferred stock has redemption features.

Derivative Financial Instruments

We do not use derivative instruments to hedge exposures to cash flow, market, or foreign currency risks. We evaluate all of our financial instruments, including warrants, to determine if such instruments are derivatives or contain features that qualify as embedded derivatives. We generally use the Black-Scholes option-pricing model to value the derivative instruments at inception and subsequent valuation dates. The classification of derivative instruments, including whether such instruments should be recorded as liabilities or as equity, is re-assessed at the end of each reporting period.

Recent Accounting Pronouncements

In January 2016, the FASB issued ASU No. 2016-01, Financial Instruments-Overall: Recognition and Measurement of Financial Assets and Financial Liabilities, which requires that (i) all equity investments, other than equity-method investments, in unconsolidated entities generally be measured at fair value through earnings and (ii) when the fair value option has been elected for financial liabilities, changes in fair value due to instrument-specific credit risk will be recognized separately in other comprehensive income. Additionally, ASU 2016-01 changes the disclosure requirements for financial instruments. The new standard will be effective for fiscal years beginning after December 15, 2017, and interim periods within those annual fiscal years. Early adoption is permitted for certain provisions. The adoption of ASU 2016-01 is not expected to have a material impact on our consolidated financial position or results of operations.

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), to increase transparency and comparability among organizations by requiring recognition of lease assets and lease liabilities on the balance sheet and disclosure of key information about leasing arrangements. The new standard will be effective for interim and annual periods beginning after December 15, 2018, with early adoption permitted, and is required to be adopted at the earliest period presented using a modified retrospective approach. We are currently evaluating the impact the provisions will have on our consolidated financial position or results of operations and whether we will adopt the guidance early.

In May 2017, the FASB issued ASU 2017-09, Compensation-Stock Compensation (Topic 718): Scope of Modification Accounting, to provide guidance about which changes to the terms or conditions of a share-based payment award require an entity to apply modification accounting in Topic 718. ASU 2017-09 will be effective for the annual periods beginning after December 15, 2017. Early adoption is permitted. We do not expect that the adoption of ASU 2017-09 will have a material impact on our consolidated financial position or results of operations.

Fair Value Measurements

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Fair Value Measurements

3. Fair Value Measurements

The carrying amounts of cash equivalents, prepaid and other assets, accounts payable and accrued liabilities are considered to be representative of their respective fair values because of the short-term nature of those instruments. Based on the borrowing rates currently available to us for loans with similar terms, which is considered a Level 2 input, we believe that the fair value of our Term Loans approximate its carrying values. Investment securities are recorded at fair value.

The accounting guidance defines fair value, establishes a consistent framework for measuring fair value and expands disclosure for each major asset and liability category measured at fair value on either a recurring or nonrecurring basis. Fair value is defined as an exit price, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. As such, fair value is a market-based measurement that should be determined based on assumptions that market participants would use in pricing an asset or liability. As a basis for considering such assumptions, the accounting guidance establishes a three-tier fair value hierarchy, which prioritizes the inputs used in measuring fair value as follows:

Level 1: Observable inputs such as quoted prices in active markets.

Level 2: Inputs, other than the quoted prices in active markets that are observable either directly or indirectly.

Level 3: Unobservable inputs in which there is little or no market data, which require the reporting entity to develop its own assumptions.

Financial assets measured at fair value on a recurring basis consist of investment securities. Investment securities are recorded at fair value, defined as the exit price in the principal market in which we would transact, representing the amount that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants. Level 2 securities are valued using quoted market prices for similar instruments, non-binding market prices that are corroborated by observable market data, or discounted cash flow techniques and include our investments in asset-backed securities, commercial paper, and corporate debt securities. We have no financial liabilities measured at fair value on a recurring basis. None of our non-financial assets and liabilities is recorded at fair value on a non-recurring basis. No transfers between levels have occurred during the periods presented.

Assets and liabilities measured at fair value on a recurring basis are as follows (in thousands):

			Fair Value Measurements Using
			Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
As of December 31, 2017:
Assets:
Current:
Cash equivalents	$	9,070	$	9,070	$	—	$	—
Available-for-sale investments, short-term:
Asset-backed securities		6,497		—		6,497		—
Commercial paper		21,943		—		21,943		—
Corporate debt securities		18,260		—		18,260		—
United States Treasury securities		17,328		17,328		—
Sub-total short-term investments		64,028		17,328		46,700		—
Total assets measured at fair value	$	73,098	$	26,398	$	46,700	$	—
As of December 31, 2016:
Assets:
Current:
Cash equivalents	$	29,251	$	29,251	$	—	$	—
Available-for-sale investments, short-term:
Commercial paper		7,843		—		7,843		—
Corporate debt securities		20,913		—		20,913		—
United States Treasury securities		5,003		5,003		—		—
Sub-total short-term investments		33,759		5,003		28,756		—
Available-for-sale investments, long-term:
Asset-backed securities		4,002		—		4,002		—
Sub-total long-term investments		4,002		—		4,002		—
Total assets measured at fair value	$	67,012	$	34,254	$	32,758	$	—

As of December 31, 2017 and 2016, available-for-sale investments are detailed as follows (in thousands):

	December 31, 2017
	Gross Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Market Value
Available-for-sale investments, short-term:
Asset-backed securities	$	6,501		—		(4	)	$	6,497
Commercial paper		21,943		—		—			21,943
Corporate debt securities		18,286		—		(26	)		18,260
United States Treasury securities		17,368		—		(40	)		17,328
	$	64,098	$	—	$	(70	)	$	64,028

	December 31, 2016
	Gross Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Market Value
Available-for-sale investments, short-term:
Commercial paper	$	7,843	$	—	$	—		$	7,843
Corporate debt securities		20,942		—		(29	)		20,913
United States Treasury securities		5,002		1		—			5,003
	$	33,787	$	1	$	(29	)	$	33,759
Available-for-sale investments, long-term:
Asset-backed securities	$	4,001	$	1	$	—		$	4,002
	$	4,001	$	1	$	—		$	4,002

As of December 31, 2017, all available-for-sale investments have contractual maturity dates less than one year. As of December 31, 2017, there are 20 available-for-sale investments in gross unrealized loss position, all of which have been in such position for less than twelve months.

At each reporting date, we perform an evaluation of impairment to determine if the unrealized losses are other-than-temporary. Factors considered in determining whether a loss is other-than-temporary include the length of time and extent to which fair value has been less than the cost basis, the financial condition of the issuer, and our intent and ability to hold the investment until recovery of its amortized cost basis. We intend, and have the ability, to hold our investments in unrealized loss positions until their amortized cost basis has been recovered. Based on our evaluation, we determined that the unrealized losses were not other-than-temporary as of December 31, 2017.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Balance Sheet Details

12 Months Ended

Dec. 31, 2017

Balance Sheet Details [Abstract]

Balance Sheet Details

4. Balance Sheet Details

Property and equipment consist of the following (in thousands):

	December 31,
	2017			2016
Computer and office equipment	$	425		$	401
Scientific and laboratory equipment		5,494			3,965
Tenant improvements		1,706			1,687
		7,625			6,053
Less accumulated depreciation and amortization		(5,345	)		(4,632	)
	$	2,280		$	1,421

Accrued expenses consist of the following (in thousands):

	December 31,
	2017		2016
Accrued salaries, wages and benefits		1,920		1,977
Other accrued expenses (1)		1,183		3,473
	$	3,103	$	5,450

(1)

Other accrued expenses include expenses for clinical research organizations and contract manufacturing organizations.

Debt, Commitments and Contingencies

12 Months Ended

Dec. 31, 2017

Debt Instruments [Abstract]

Debt, Commitments and Contingencies

5. Debt, Commitments and Contingencies

Term Loans

In November 2016, we entered into a loan and security agreement (the Loan Agreement) with Silicon Valley Bank (SVB) and Solar Capital Ltd. (Solar) to borrow up to $20.0 million, issuable in three separate tranches of $10.0 million, $5.0 million and $5.0 million, respectively. The first tranche of $10.0 million was funded in November 2016 (Term A Loan). Under the Term A Loan, we received cash proceeds of $7.3 million, net of a $2.6 million repayment of the principal, accrued interest and the $0.5 million final payment under our previous $10.0 million loan and security agreement with SVB (SVB Loan). We did not pay any termination or other fees in connection with the repayment of amounts due under the SVB Loan.

In June 2017 and December 2017, we entered amendments to the Loan Agreement which modified certain conditions under which we may receive and repay the term loans under the Loan Agreement. The second (Term B) and third (Term C) tranche of $5.0 million each were funded in June 2017 and December 2017, respectively. We received total cash proceeds of $9.9 million, net of debt issuance costs of $0.1 million for both tranches. The amendments to the Loan Agreement were not a result of a troubled debt restructuring and did not result in modifications to the terms that were substantially different.

Under the Loan Agreement, we were originally obligated to make interest only payments through December 1, 2017, followed by consecutive equal monthly payments of principal and interest in arrears through the maturity date of November 18, 2020. With the Loan Agreement, as amended, the interest only period was extended to June 1, 2018. The Term A Loan, Term B Loan and Term C Loan (collectively, the Term Loans) bear interest at the prime rate, as reported in The Wall Street Journal on the last date of the month preceding the month in which interest will accrue, plus 4.10%. A final payment equal to 8.75% of the funded amounts is payable when the Term Loans become due or upon the prepayment of the respective outstanding balance. We have the option to prepay the outstanding balance of the loan in full, subject to a prepayment fee ranging from 1.0% to 3.0% depending upon when the prepayment occurs, as well as any non-usage fees.

The obligations under the Term Loans are secured by liens on our tangible personal property and we agreed to not encumber any of our intellectual property. The Term Loans include a material adverse change clause, which enables the Lenders to require immediate repayment of the outstanding debt. The material adverse change clause covers a material impairment in the perfection or priority of the lenders’ lien in the underlying collateral or in the value of such collateral, material adverse change in business operations or condition or material impairment of our prospects for repayment of any portion of the remaining debt obligation.

As of December 31, 2017, the carrying value of our Term Loans consist of $20.0 million principal outstanding less the debt issuance costs of $0.9 million. The debt issuance costs have been recorded as a debt discount which are being accreted to interest expense over the life of the Term Loans. The final maturity payment of $1.8 million is accruing over the life of the Term Loans through interest expense.

In connection with the Term A Loan, we issued warrants to each of SVB and Solar to purchase an aggregate of 47,771 shares of our common stock with an exercise price of $3.14 per share. In connection with the Term B Loan, we issued warrants to each of SVB and Solar to purchase an aggregate of 20,833 shares of our common stock with an exercise price of $3.60 per share. In connection with the Term C Loan, we issued warrants to each of SVB and Solar to purchase an aggregate of 20,188 shares of our common stock with an exercise price of $3.72 per share. The warrants are immediately exercisable and have a maximum contractual term of seven years. The aggregate fair value of the warrants was determined to be $0.5 million using the Black-Scholes option pricing model and was recorded as debt discount which are being accreted to interest expense over the life of Term Loans.

Term loans and unamortized discount balances are as follows (in thousands):

	December 31,
	2017			2016
Long-term debt	$	20,000		$	10,000
Less debt issuance costs and discount		(345	)		(463	)
Long-term debt, net of issuance costs and discount		19,655			9,537
Less current portion of long-term debt		(5,333	)		(339	)
Add accrual of final payment		397			—
Long-term debt, net of current portion and issuance costs and discount	$	14,719		$	9,198

Current portion of long-term debt	$	5,333		$	339
Less current portion of debt issuance costs and discount		(321	)		—
Current portion of long-term debt, net of issuance costs and discount	$	5,012		$	339

Future principal payments for the Term Loans are as follows (in thousands):

	December 31, 2017
2018	$	5,333
2019		8,000
2020		6,667
	$	20,000

Facility Lease

In December 2011, we entered into a noncancelable operating lease that included certain tenant improvement allowances and is subject to base lease payments, which escalate over the term of the lease, additional charges for common area maintenance and other costs. In January 2017, we extended the lease for two years to May 2019.

In April 2017, we amended our facility lease to include an additional 7,411 square feet through May 2019 for a total additional commitment of $0.7 million.

Rent expense for the years ended December 31, 2017, 2016 and 2015 was $0.9 million, $0.5 million and $0.4 million, respectively.

As of December 31, 2017, future minimum payments under the non-cancelable operating lease are as follows (in thousands):

	Operating Lease
2018	$	1,108
2019		420
	$	1,528

Research Agreements and Funding Obligations (Related Party Transactions)

We provide funding to The Scripps Research Institute (TSRI) pursuant to a research funding and option agreement to conduct certain research activities. We have entered into additional amendments to our research funding and option agreement in 2016 and 2017 to provide additional funding to TSRI. During the years ended December 31, 2017, 2016 and 2015, excluding the fair value of the common stock issued to TSRI, we recognized expense under the agreement in the amount of $1.8 million, $1.6 million and $0.7 million, respectively. Paul Schimmel, Ph.D., a member of our board of directors, is a board and faculty member at TSRI and such payments fund a portion of his research activities conducted at TSRI.

During the years ended December 31, 2017, 2016 and 2015, we provided charitable donations to the National Foundation for Cancer Research of $0.2 million, $0.4 million and $0.4 million, respectively. We have requested that the donations be restricted to certain basic research in cancer biology and therapeutics, a portion of which funds research activities conducted at TSRI in the laboratory of a member of our board of directors.

Manufacturing Agreements

In August 2016, we entered into a Master Services Agreement with a third party contract development and manufacturing organization to complete the development of the manufacturing process and for the production of drug substance for ATYR1923. We are required to pay the third party contract manufacturer a total payment in the low seven figures subject to certain rights of cancellation. In addition, we are billed for consumables on a pass-through basis. In the next 12 months, based on statements of work, we are committed to pay the third party contract manufacturing organization approximately $1.6 million based on development and production milestones. For the year ended December 31, 2017 and 2016, expenses associated with this agreement were $2.8 million and $1.1 million, respectively.

Stockholders' Equity

12 Months Ended

Dec. 31, 2017

Equity [Abstract]

Stockholders' Equity

6. Stockholders’ Equity

Common Stock

Private Placement of Common Stock, Convertible Preferred Shares and Common Stock Warrants

On August 27, 2017, we entered into a Securities Purchase Agreement (Securities Purchase Agreement) for a private placement (Private Placement) with a select group of institutional investors, including Viking Global Opportunities Illiquid Investments Sub-Master, LP (VGO Fund) and other accredited investors, certain of whom are affiliated with our directors and officers (collectively, the Purchasers). Pursuant to the Securities Purchase Agreement, (i) VGO Fund purchased 1,777,784 shares of our common stock, par value $0.001 per share (the Common Shares), at a price of $2.65 per share, 2,285,952 shares of our Class X Convertible Preferred Stock (the Preferred Shares or Preferred Stock, and together with the Common Shares, the Shares), par value $0.001 per share, at a price of $13.25 per share, and warrants to purchase up to that number of additional shares of Common Stock equal to thirty seven and one half percent (37.5%) of the number of Shares purchased by VGO Fund on an if-converted to common stock basis (rounded up to the nearest whole share), and (ii) the remaining Purchasers purchased an aggregate of 4,094,336 shares of our Common Shares, at a price of $2.65 per share, and warrants to purchase up to that number of additional shares of Common Stock equal to thirty-seven and one half percent (37.5%) of the number of Common Shares purchased by such Purchaser (rounded up to the nearest whole share). The Private Placement closed on August 31, 2017 for gross proceeds of $45.8 million, and after giving effect to costs related to the Private Placement, net proceeds of $42.5 million.

Each share of Preferred Stock is convertible into five shares of our common stock. VGO Fund will be prohibited from converting the Preferred Stock into shares of our common stock if, as a result of such conversion, VGO Fund, together with its affiliates, would own more than 9.50% of the shares of our common stock then issued and outstanding, which percentage may change at VGO Fund’s election upon 61 days’ notice to us to (i) any other number less than or equal to 19.99% or (ii) subject to approval of our stockholders to the extent required in accordance with the NASDAQ Global Market rules, any number in excess of 19.99%.

Holders of outstanding Preferred Stock are entitled to receive a dividend (on an if-converted to common stock basis), if we at any time pay a stock dividend equal to and in the same form as a dividend paid to holders of Common Shares.

In the event of our liquidation, dissolution or winding up, holders of Preferred Stock will participate in any distribution of proceeds, pro rata based on the number of shares held by each such holder on an if-converted basis. The Preferred Shares have no voting rights.

We evaluated the Preferred Stock for liability or equity classification under ASC 480, Distinguishing Liabilities from Equity (ASC480), and determined that equity treatment was appropriate because the Preferred Stock did not meet the definition of the liability instruments defined thereunder for convertible instruments. Specifically, the Preferred Stock are not mandatorily redeemable and do not embody an obligation to buy back the shares outside of our control in a manner that could require the transfer of assets. Additionally, we determined that the Preferred Stock would be recorded as permanent equity, not temporary equity, based on the guidance of ASC 480 given that they are not redeemable for cash or other assets (i) on a fixed or determinable date, (ii) at the option of the holder, and (iii) upon the occurrence of an event that is not solely within control of the Company.

We also evaluated the Preferred Stock in accordance with the provisions of ASC 815, Derivatives and Hedging, including the consideration of embedded derivatives requiring bifurcation from the equity host. Based on this assessment, we determined that the conversion option is closely related to the equity host, and thus, bifurcation is not required.

The issuance of convertible preferred stock could generate a beneficial conversion feature (BCF), which arises when a debt or equity security is issued with an embedded conversion option that is beneficial to the investor (or in-the-money) at inception because the conversion option has an effective strike price that is less than the market price of the underlying stock on the commitment date. The fair value of our common stock was $2.37 on August 31, 2017, the commitment date, using the Black-Scholes valuation model. After the proceeds allocation, the Preferred Stock have an effective conversion price of $2.37 per common share, which was equal to the fair value of our common stock on the commitment date. Therefore, no BCF is present.

The warrants are exercisable at an exercise price of $4.64 per share, subject to adjustments as provided under the terms of the warrants. The warrants are immediately exercisable and expire on December 31, 2019. We also entered into a registration rights agreement (Registration Rights Agreement) with certain of the Purchasers, excluding those Purchasers affiliated with our directors and officers, requiring us to register for the resale of the relevant securities. We registered all of the relevant securities issued in the Private Placement for resale on a Form S-3 filed with the SEC, as required under the Registration Rights Agreement, and the registration statement was declared effective on September 27, 2017.

We evaluated the warrants for liability or equity classification under ASC 815, Derivative and Hedging (ASC 815) and determined that equity treatment was appropriate because the warrants are indexed to our common stock and no cash settlement is required except for (i) liquidation of the Company, or (ii) a change in control in which the common stockholders also received cash.

Registration Statement on Form S-3

In June 2016, we filed a Registration Statement on Form S-3 (File No. 333-211998) containing two prospectuses: (i) a base prospectus which covers the offering, issuance and sale of up to $150 million in the aggregate of an indeterminate number of shares of common stock and preferred stock, an indeterminate principal amount of debt securities and such indeterminate number of warrants and units; and (ii) a sales agreement prospectus covering the offering, issuance and sale of up to a maximum aggregate offering price of up to $20 million of our common stock that may be sold from time to time under a sales agreement with Cowen and Company, LLC (Cowen). In accordance with the terms of such sales agreement entered with Cowen, we may offer and sell shares of our common stock having an aggregate offering price of up to $35 million from time to time through Cowen. We are required to file another prospectus supplement in the event we intend to offer more than $20 million in shares of our common stock in accordance with the sales agreement. The sales agreement prospectus amount of $20 million is included in the base prospectus amount of $150 million.

2014 Stock Plan

We adopted a stock option plan in 2007 (the 2007 Plan), which was subsequently amended, restated and renamed in July 2014 (the 2014 Plan) to provide for the incentive stock options, nonstatutory stock options, stock and rights to purchase restricted stock to eligible recipients. Recipients of incentive stock options are eligible to purchase shares of our common stock at an exercise price equal to no less than the estimated fair market value of such stock on the date of grant. The maximum term of options under the 2014 Plan is ten years. Options granted generally vest over four years. Shares underlying any awards under the 2014 Plan that are forfeited, canceled, reacquired by us prior to vesting, satisfied without the issuance of stock or otherwise terminated (other than by exercise) will be added to shares available for issuance under the 2015 Plan.

2015 Stock Plan

In April 2015, our board of directors adopted, and our stockholders approved, the 2015 Stock Plan (the 2015 Plan). The 2015 Plan became effective on May 6, 2015 and we ceased granting any new awards under our 2014 Plan. Awards granted under the 2014 Plan prior to our IPO that are forfeited, canceled, reacquired by us prior to vesting satisfied without the issuance of stock or otherwise terminated (other than by exercise) will be added to shares available for issuance under the 2015 Plan. A total of 1,574,566 shares of our common stock were initially reserved for issuance under the 2015 Plan. In addition, the number of shares reserved and available for issuance under the 2015 Plan will automatically increase each January 1, beginning on January 1, 2016 and thereafter until January 1, 2019, by the lesser of (i) 1,840,000 shares, (ii) 4% of the outstanding number of shares of our common stock on the immediately preceding December 31 or (iii) an amount determined by our board of directors. Pursuant to this provision, 1,191,566, 949,793 and 946,803 additional shares were reserved for issuance under the 2015 Plan on January 1, 2018, 2017 and 2016, respectively. Shares underlying any awards under the 2015 Plan that are forfeited, canceled, reacquired by us prior to vesting, satisfied without the issuance of stock or otherwise terminated (other than by exercise) will be added to shares available for issuance under the 2015 Plan.

The maximum term of options granted under 2015 Plan is ten years. For an initial grant to an employee, 25% of the options generally vest on the first anniversary of the original vesting date, with the balance vesting monthly over the remaining three years. For subsequent grants to an employee, the options generally vest monthly over a four-year term.

Inducement Grant

In September 2016, we granted a non-qualified option to purchase 145,000 shares of our common stock at an exercise price of $3.29 per share as an inducement award in connection with the hiring of our Senior Vice President, Research. This option will vest over a period of four (4) years, with 25% vesting on the one year anniversary of the grant date and the remaining 75% vesting on a monthly basis over three years thereafter, subject to continuous employment. This option was an inducement grant issued outside of the 2015 Plan in accordance with NASDAQ Listing Rule 5635(c)(4). We filed a registration statement on Form S-8 on March 22, 2017 to register the shares of common stock underlying this option. In addition, from time to time, we may make inducement grants of stock options to new employees. There were no inducement awards granted in 2017.

Employee Stock Purchase Plan

In April 2015, our board of directors adopted, and our stockholders approved, our 2015 Employee Stock Purchase Plan (the 2015 ESPP). The 2015 ESPP became effective on May 6, 2015. A total of 227,623 shares of our common stock were initially reserved for issuance under the 2015 ESPP. In addition, the number of shares reserved and available for purchase under the 2015 ESPP will automatically increase each January 1, beginning on January 1, 2016 and thereafter until January 1, 2019, by 1% of the outstanding number of shares of our common stock on the immediately preceding December 31 or such lesser number of shares as determined by the administrator of the 2015 ESPP. Pursuant to this provision, 297,891, 237,448 and 236,700 additional shares were reserved for issuance under the 2015 ESPP on January 1, 2018, 2017 and 2016, respectively.

Stock-based Compensation

Stock Options

Stock option activity is summarized as follows:

	Number of Outstanding Options			Weighted Average Exercise Price		Weighted Remaining Contractual Term		Aggregate Intrinsic Value (in 000s)
Outstanding as of December 31, 2016		4,014,988		$	6.73
Granted		1,508,119		$	3.51
Exercised		(75,289	)	$	2.47
Canceled/forfeited/expired		(830,759	)	$	7.99
Outstanding as of December 31, 2017		4,617,059		$	5.52		7.37	$	1,370
Options vested and expected to vest as of December 31, 2017		4,617,059		$	5.52		7.37	$	1,370
Options exercisable as of December 31, 2017		2,220,670		$	6.01		6.23	$	1,070

The assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants were as follows:

	Years Ended December 31,
	2017			2016			2015
Expected term (in years)	5.50 – 6.08			5.50 – 6.08			5.50 – 6.08
Risk-free interest rate	1.9% – 2.1		%	1.2% – 2.1		%	1.5% – 1.9		%
Expected volatility	99.1% – 124.4		%	80.7% – 84.5		%	79.2% – 100.9		%
Expected dividend yield		0.0	%		0.0	%		0.0	%

The assumptions used in the Black-Scholes option pricing model to determine the fair value of the ESPP offering were as follows:

	Years Ended December 31,
	2017			2016			2015
Expected term (in years)		0.50			0.50			0.50
Risk-free interest rate	0.6% – 1.0		%	0.4% – 0.6		%		0.3	%
Expected volatility	74.5% – 115.2		%	75.5% – 80.8		%		67.3	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

Expected term. The expected term represents the period of time that options are expected to be outstanding. Because we do not have sufficient history of exercise behavior, we determine the expected life assumption using the simplified method, which is an average of the contractual term of the option and its vesting period.

Risk-free interest rate. We base the risk-free interest rate assumption on the U.S. Treasury’s rates for U.S. Treasury zero-coupon bonds with maturities similar to those of the expected term of the award being valued.

Expected volatility. The expected volatility assumption is based on volatilities of a peer group of similar companies whose share prices are publicly available. The peer group was developed based on companies in the biotechnology industry.

Expected dividend yield. We base the expected dividend yield assumption on the fact that we have never paid cash dividends and have no present intention to pay cash dividends.

Performance Options with Market Conditions

In October 2015, we granted to our executives, employees and certain consultants performance options with a market condition to purchase up to an aggregate 169,402 shares of common stock at an exercise price of $10.24. Upon achievement of specified market condition by October 2017, such performance options shall begin to vest over four years in equal monthly installments, otherwise the options will be subject to forfeiture. The fair value of the performance options with market conditions is estimated on the date of the grant using a Monte Carlo simulation, based on the market price of the underlying common stock, expected performance measurement period, expected peer group stock price volatility and expected risk-free interest rate. The weighted average grant date fair value was $4.23. The performance options with market conditions grants are expensed using the accelerated attribution method over the requisite service period of 4.8 years regardless of whether the market condition is achieved or earned and vested. As of October 2017, the market condition for these performance options were not met and therefore were forfeited.

In January 2016, we granted to our executives, employees and certain consultants performance options with a market condition to purchase up to an aggregate 396,960 shares of common stock at an exercise price of $9.13. Upon achievement of specified market conditions by January 2018, such performance options shall begin to vest over four years in equal monthly installments, otherwise the options will be subject to forfeiture. The fair value of the performance options with a market condition is estimated on the date of the grant using a Monte Carlo simulation, based on the market price of the underlying common stock, expected performance measurement period, expected peer group stock price volatility and expected risk-free interest rate. The weighted average grant date fair value was $1.93. The performance options with market conditions grants are expensed using the accelerated attribution method over the requisite service period of 5.1 years regardless of whether the market condition is achieved or earned and vested. Refer to Note 10, Subsequent Events for further discussion of these options.

There were no performance options with a market condition granted during 2017.

The assumptions used at grant date to determine the fair value of the performance options with a market condition were as follows:

	December 31,
	2016			2015
Expected term (in years)		5.06			4.81
Risk-free interest rate		2.2	%		2.1	%
Expected volatility		83.3	%		80.6	%
Expected dividend yield		0.0	%		0.0	%

Restricted Stock Units

During the year ended December 31, 2017, we granted restricted stock units to employees. Restricted stock unit activity is summarized as follows:

	Number of Outstanding Restricted Stock Units			Weighted Average Grant Date Fair Value
Balance as of December 31, 2016		76,713		$	4.95
Granted		22,000		$	3.30
Released		(36,099	)	$	4.96
Forfeited		(13,314	)	$	4.67
Balance as of December 31, 2017		49,300		$	4.28

The allocation of stock-based compensation for all options, including performance options with market condition and restricted stock units is as follows (in thousands):

	Years Ended December 31,
	2017		2016		2015
Research and development	$	1,399	$	1,876	$	2,524
General and administrative		5,385		3,153		2,332
	$	6,784	$	5,029	$	4,856

The weighted–average grant date fair value per share of stock options granted by us, excluding performance options with market conditions, during the years ended December 31, 2017, 2016 and 2015 was $2.85, $3.34 and $11.29, respectively. The total fair value of restricted stock units vested during the years ended December 31, 2017, 2016 and 2015 was $0.1 million, $13,000 and none, respectively. The aggregate intrinsic value of stock options exercised during the years ended December 31, 2017, 2016 and 2015 was $0.3 million, $34,000 and $1.9 million, respectively. As of December 31, 2017, total unrecognized share-based compensation expense related to unvested stock options and restricted stock units was approximately $6.0 million. This unrecognized cost is expected to be recognized ratably over a weighted-average period of approximately 2.6 years.

In October 2015, our Compensation Committee of the Board of Directors approved an amendment to accelerate the vesting schedule of certain outstanding stock options representing 931,749 shares granted to active employees and certain consultants under the 2014 Plan to change the vesting schedule of such options from six-years to four-years retroactive to the original vesting commencement dates. We recorded $0.8 million of stock compensation expense in connection with the modification during the year ended December 31, 2015.

During the fourth quarter of 2017, in connection with the change of status of our then-Chief Executive Officer to an advisor consulting role, we modified certain terms of outstanding options granted to the executive. We recorded $1.9 million of share-based compensation expense related to the modifications. We determined that vesting of the shares underlying the options will occur whether or not our then-Chief Executive Officer provides substantive service. In addition, in connection with the departure of our then-Chief Business Officer, we modified certain terms of outstanding options previously granted to the executive. As a result, we recorded $0.3 million in share-based compensation expense related to the modification.

Warrants

Warrants outstanding as of December 31, 2017 is as follows:

Number		Exercise Price		Expiration
Outstanding		Per Share		Date
	6,488,205	$	4.64	December 2019
	2,006	$	7.48	March 2021
	14,913	$	20.12	July 2023
	95,542	$	3.14	November 2023
	41,666	$	3.60	June 2024
	40,376	$	3.72	December 2024
	6,682,708

Common Stock Reserved for Future Issuance

Common stock reserved for future issuance is as follows:

	December 31,
	2017		2016
Class X Preferred Stock (if-converted to common stock)		11,429,760		—
Common stock warrants		6,682,708		121,512
Common stock options and awards outstanding		4,666,359		4,091,701
Shares available under the 2015 Plan		765,427		510,760
Shares available under the 2015 ESPP		583,819		407,542
		24,128,073		5,131,515

Income Tax

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Income Tax

7. Income Tax

Pretax earnings (loss) were generated by both domestic and foreign operations as follows (in thousands):

	Years Ended December 31,
	2017			2016			2015
United States	$	(47,712	)	$	(57,096	)	$	(47,490	)
Foreign		(495	)		(808	)		(483	)
	$	(48,207	)	$	(57,904	)	$	(47,973	)

A reconciliation of the expected statutory federal income tax provision to the actual income tax provision is summarized as follows (in thousands):

	Years Ended December 31,
	2017			2016			2015
Expected income taxes benefit at federal statutory rate	$	(16,390	)	$	(19,687	)	$	(16,311	)
State income taxes, net of federal benefit		(13	)		—			—
Permanent items and other		1,311			675			865
Research credits		(2,286	)		(6,800	)		(2,674	)
Unrecognized tax benefits		914			2,720			1,070
Foreign rate differential		87			141			84
Change in tax rate		(25	)		—			3,551
Tax cuts and Jobs Act		27,933			—			—
Change in valuation allowance		(11,531	)		22,902			13,415
Income tax (benefit) expense	$	—		$	(49	)	$	—

Deferred income taxes are provided for temporary differences in recognizing certain income and expense items for financial and tax reporting purposes. The deferred tax assets consisted primarily of the income tax benefits from net operating loss (NOL) carryforwards, research and development credits and capitalized research and development expenses, along with other accruals and reserves. Valuation allowances of $59.7 million and $71.1 million as of December 31, 2017 and 2016, respectively, have been recorded to offset deferred tax assets as realization of such assets does not meet the more-likely-than-not threshold under ASC 740, Accounting for Income Taxes.

Significant components of our deferred tax assets are summarized as follows (in thousands):

	December 31,
	2017			2016
Net operating loss carryforwards	$	27,226		$	33,713
Capitalized research and development expenses		16,218			21,624
Research credits and other state credits		11,229			9,227
Intangible assets		2,210			3,874
Reserve and accruals		2,843			2,711
Valuation allowance		(59,726	)		(71,149	)
Net deferred tax assets	$	—		$	—

As of December 31, 2017, we had approximately $112.5 million, $119.2 million, and $7.0 million of net operating loss carryforwards for federal, state, and foreign purposes, respectively, net of Section 382 limitations, available to offset future taxable income. The federal and state net operating loss carryforwards begin to expire in 2025 and 2021, respectively. The foreign net operating losses carry over indefinitely. As of December 31, 2017, we had federal and state research and development credit carryforwards of approximately $3.6 million and $3.2 million, respectively, net of Section 382 limitations, which begin to expire in 2026 for federal purposes and carry over indefinitely for state purposes. We had $12.5 million of federal Orphan Drug Credits as of December 31, 2017, which will begin to expire in 2035.

Utilization of the domestic NOL and research and development credit carryforwards may be subject to a substantial annual limitation due to ownership change limitations that may have occurred or that could occur in the future, as required by Section 382 and 383 of the Internal Revenue Code of 1986, as amended (the Code), as well as similar state and foreign provisions. These ownership changes may limit the amount of NOL and research and development credit carryforwards that can be utilized annually to offset future taxable income and tax, respectively. In general, an “ownership change” as defined by Section 382 of the Code results from a transaction or series of transactions over a three-year period resulting in an ownership change of more than 50 percentage points of the outstanding stock of a company by certain stockholders. Since the Company’s formation, we raised capital through the issuance of capital stock on several occasions which on its own or combined with the purchasing stockholders’ subsequent disposition of those shares, has resulted in such an ownership change, and could result in an ownership change in the future.

Upon the occurrence of an ownership change under Section 382 as outlined above, utilization of the NOL and research and development credit carryforwards become subject to an annual limitation under Section 382 of the Code, which is determined by first multiplying the value of our stock at the time of the ownership change by the applicable long-term, tax-exempt rate, which could be subject to additional adjustments. Any limitation may result in expiration of a portion of the NOL or research and development credit carryforwards before utilization. We completed an analyses through December 31, 2017, and are in the process of analyzing the impact to our NOL and research and development tax credit carryforwards. Due to the existence of the valuation allowance, any impact to the NOL and R&D tax credit carryforwards from Section 382 analysis will be offset by a corresponding adjustment to valuation allowance, resulting no tax provision impact. Ownership changes that may have occurred subsequent to December 31, 2017, and future ownership changes, including any ownership change resulting from this offering, may further limit our ability to utilize its remaining tax attributes.

We recognize a tax benefit from an uncertain tax position when it is more likely than not that the position will be sustained upon examination, including resolutions of any related appeals or litigation processes, based on the technical merits. Income tax positions must meet a more-likely-than-not recognition threshold to be recognized.

Our practice is to recognize interest and penalties related to income tax matters in income tax expense. We had no accrual for interest and penalties on our balance sheet and had not recognized interest or penalties in the consolidated statements of operations for the years ended December 31, 2017, 2016 and 2015.

Due to the existence of the valuation allowance, future changes in unrecognized tax benefits will not impact our effective tax rate.

Uncertain tax positions are evaluated based upon the facts and circumstances that exist at each reporting period. Subsequent changes in judgment based upon new information may lead to changes in recognition, derecognition, and measurement. Adjustments may result, for example, upon resolution of an issue with the taxing authorities, or expiration of a statute of limitations barring an assessment for an issue.

The activity related to our unrecognized tax benefits is summarized as follows (in thousands):

	December 31,
	2017			2016		2015
Balance as of beginning of year	$	13,000		$	5,033	$	1,106
Increase (decrease) related to prior year tax positions		(189	)		1,890		2,404
Increase related to current year tax positions		3,747			6,077		1,523
Balance as of end of year	$	16,558		$	13,000	$	5,033

We do not anticipate that the amount of unrecognized tax benefits as of December 31, 2017 will change within the next twelve months.

We are subject to taxation in the United States, Hong Kong and state jurisdictions. Our tax years from inception are subject to examination by the United States, Hong Kong and California authorities due to the carry forward of unutilized NOLs and research and development credits.

The Tax Cuts and Jobs Act (the Act) was enacted on December 22, 2017. Among other changes, the Act reduces the U.S. federal corporate tax rate from 34 percent to 21 percent. In accordance with Staff Accounting Bulletin 118, as of December 31, 2017, we have not completed our accounting for the tax effects of enactment of the Act; however, in certain cases, as described below, we have made a reasonable estimate of the effects on our existing deferred tax balances. In all cases, we will continue to make and refine our calculations as additional analysis is completed. In addition, our estimate may also be effected as we gain a more thorough understanding of the tax law.

We remeasured certain deferred tax assets and liabilities based on the rates at which they are expected to reverse in the future, which is generally 21 percent. However, we are still analyzing certain aspects of the Act and refining our calculations, which could potentially affect the measurement of these balances or give rise to new deferred tax amounts. The provisional amount recorded related to the remeasurement of our deferred tax balance was $27.9 million, which was fully offset by a decrease in our valuation allowance.

Due to uncertainties which currently exist in the interpretation of the provisions of the Act regarding Internal Revenue Code (IRC) Section 162(m), the Company is continuing to evaluate the potential impacts of IRC Section 162(m) as amended by the Act in its financial statements.

Employee Benefits	12 Months Ended
Employee Benefits	Dec. 31, 2017
Compensation And Retirement Disclosure [Abstract]
Employee Benefits	8. Employee Benefits 401(k) Plan We maintain a defined contribution 401(k) plan available to eligible employees. Employee contributions are voluntary and are determined on an individual basis, limited to the maximum amount allowable under federal tax regulations. In April 2015, our Board of Directors approved a policy, beginning on June 1, 2015, to match employee contributions equal to 50% of the participant’s contribution of up to a maximum of 6% of the participant’s annual salary. We made discretionary contributions totaling $0.2 million, $0.2 million and $0.1 million during the years ended December 31, 2017, 2016 and 2015, respectively.

Quarterly Financial Data (Unaudited)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Data [Abstract]

Quarterly Financial Data (Unaudited)

9. Quarterly Financial Data (Unaudited)

The following financial information reflects all normal recurring adjustments, which are, in our opinion, necessary for a fair statement of the results of the interim periods. Summarized quarterly data for 2017 and 2016 are as follows (in thousands, except per share data):

	For the quarters ended
	March 31			June 30			September 30			December 31
2017:
Operating expenses	$	13,211		$	11,907		$	10,827		$	11,200
Net loss		(13,405	)		(12,138	)		(11,190	)		(11,474	)
Basic and diluted net loss per share	$	(0.56	)	$	(0.51	)	$	(0.43	)		(0.39	)

2016:
Operating expenses	$	16,115		$	15,433		$	13,865		$	12,527
Net loss		(16,087	)		(15,383	)		(13,819	)		(12,566	)
Basic and diluted net loss per share	$	(0.68	)	$	(0.65	)	$	(0.58	)		(0.53	)

Net loss per share is computed independently for each of the quarters presented. Therefore, the sum of the quarterly per-share calculations will not necessarily equal the annual per share calculation.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Organization and Business

We were incorporated in the state of Delaware on September 8, 2005. We are focused on the discovery and clinical development of innovative medicines using our knowledge of tRNA synthetase biology.

Principles of Consolidation

Our consolidated financial statements include our accounts, our 98% majority-owned subsidiary in Hong Kong, Pangu BioPharma Limited (Pangu BioPharma). All intercompany transactions and balances are eliminated in consolidation.

Use of Estimates

Our consolidated financial statements are prepared in accordance with generally accepted accounting principles (GAAP). The preparation of our consolidated financial statements requires us to make estimates and assumptions that impact the reported amounts of assets, liabilities and expenses and the disclosure of contingent assets and liabilities in our consolidated financial statements and accompanying notes. The most significant estimates in our consolidated financial statements relate to the fair value of equity issuances and awards, and clinical trials and research and development expenses. Although these estimates are based on our knowledge of current events and actions we may undertake in the future, actual results may ultimately differ materially from these estimates and assumptions.

Segment Reporting

Operating segments are identified as components of an enterprise about which separate discrete financial information is available for evaluation by the chief operating decision-maker in making decisions regarding resource allocation and assessing performance. We view our operations and manage our business in one operating segment.

Cash and Cash Equivalents

Investment Securities

Concentration of Credit Risk

Property and Equipment

Impairment of Long-Lived Assets

Accrued Expenses

Research and Development Costs

Patent Costs

Costs related to filing and pursuing patent applications are recorded as general and administrative expense and expensed as incurred since recoverability of such expenditures is uncertain.

Stock-Based Compensation

Income Taxes

Net Loss Per Share

Potentially dilutive securities not included in the calculation of diluted net loss per share because to do so would be anti-dilutive are as follows (in common share equivalents):

	Year Ended December 31,
	2017		2016		2015
Class X Convertible Preferred Stock (if-converted)		11,429,760		—		—
Warrants for common stock		6,682,708		121,512		25,970
Common stock options and restricted stock units		4,666,359		4,091,701		2,625,280
Employee stock purchase plan		31,086		36,836		17,363
		22,809,913		4,250,049		2,668,613

The following table summarizes our net loss per share (in thousands, except per share data):

	Year Ended December 31,
	2017			2016			2015
Numerator:
Consolidated net loss	$	(48,207	)	$	(57,855	)	$	(47,973	)
Accretion to redemption value		—			—			(15	)
Net loss attributable to common stockholders		(48,207	)		(57,855	)		(47,988	)

Denominator:
Weighted average common shares outstanding		25,803,538			23,707,003			15,900,167
Weighted average common shares subject to repurchase		(3,685	)		(25,984	)		(61,814	)
Weighted average common shares outstanding - basic and diluted		25,799,853			23,681,019			15,838,353

Net loss per share - basic and diluted	$	(1.87	)	$	(2.44	)	$	(3.03	)

Convertible Preferred Stock

Derivative Financial Instruments

Recent Accounting Pronouncements

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2017

Accounting Policies [Abstract]

Schedule of Potentially Dilutive Securities Not Included in Calculation of Diluted Net Loss Per Share

Potentially dilutive securities not included in the calculation of diluted net loss per share because to do so would be anti-dilutive are as follows (in common share equivalents):

	Year Ended December 31,
	2017		2016		2015
Class X Convertible Preferred Stock (if-converted)		11,429,760		—		—
Warrants for common stock		6,682,708		121,512		25,970
Common stock options and restricted stock units		4,666,359		4,091,701		2,625,280
Employee stock purchase plan		31,086		36,836		17,363
		22,809,913		4,250,049		2,668,613

Summary of Net Loss Per Share

The following table summarizes our net loss per share (in thousands, except per share data):

	Year Ended December 31,
	2017			2016			2015
Numerator:
Consolidated net loss	$	(48,207	)	$	(57,855	)	$	(47,973	)
Accretion to redemption value		—			—			(15	)
Net loss attributable to common stockholders		(48,207	)		(57,855	)		(47,988	)

Denominator:
Weighted average common shares outstanding		25,803,538			23,707,003			15,900,167
Weighted average common shares subject to repurchase		(3,685	)		(25,984	)		(61,814	)
Weighted average common shares outstanding - basic and diluted		25,799,853			23,681,019			15,838,353

Net loss per share - basic and diluted	$	(1.87	)	$	(2.44	)	$	(3.03	)

Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis

Assets and liabilities measured at fair value on a recurring basis are as follows (in thousands):

			Fair Value Measurements Using
			Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
As of December 31, 2017:
Assets:
Current:
Cash equivalents	$	9,070	$	9,070	$	—	$	—
Available-for-sale investments, short-term:
Asset-backed securities		6,497		—		6,497		—
Commercial paper		21,943		—		21,943		—
Corporate debt securities		18,260		—		18,260		—
United States Treasury securities		17,328		17,328		—
Sub-total short-term investments		64,028		17,328		46,700		—
Total assets measured at fair value	$	73,098	$	26,398	$	46,700	$	—
As of December 31, 2016:
Assets:
Current:
Cash equivalents	$	29,251	$	29,251	$	—	$	—
Available-for-sale investments, short-term:
Commercial paper		7,843		—		7,843		—
Corporate debt securities		20,913		—		20,913		—
United States Treasury securities		5,003		5,003		—		—
Sub-total short-term investments		33,759		5,003		28,756		—
Available-for-sale investments, long-term:
Asset-backed securities		4,002		—		4,002		—
Sub-total long-term investments		4,002		—		4,002		—
Total assets measured at fair value	$	67,012	$	34,254	$	32,758	$	—

Schedule of Available-for-sale Investments

As of December 31, 2017 and 2016, available-for-sale investments are detailed as follows (in thousands):

	December 31, 2017
	Gross Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Market Value
Available-for-sale investments, short-term:
Asset-backed securities	$	6,501		—		(4	)	$	6,497
Commercial paper		21,943		—		—			21,943
Corporate debt securities		18,286		—		(26	)		18,260
United States Treasury securities		17,368		—		(40	)		17,328
	$	64,098	$	—	$	(70	)	$	64,028

	December 31, 2016
	Gross Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Market Value
Available-for-sale investments, short-term:
Commercial paper	$	7,843	$	—	$	—		$	7,843
Corporate debt securities		20,942		—		(29	)		20,913
United States Treasury securities		5,002		1		—			5,003
	$	33,787	$	1	$	(29	)	$	33,759
Available-for-sale investments, long-term:
Asset-backed securities	$	4,001	$	1	$	—		$	4,002
	$	4,001	$	1	$	—		$	4,002

- Definition

Tabular disclosure of available-for-sale securities which includes, but is not limited to, changes in the cost basis and fair value, fair value and gross unrealized gain (loss), fair values by type of security, contractual maturity and classification, amortized cost basis, contracts to acquire securities to be accounted for as available-for-sale, debt maturities, transfers to trading, change in net unrealized holding gain (loss) net of tax, continuous unrealized loss position fair value, aggregate losses qualitative disclosures, other than temporary impairment (OTTI) losses or other disclosures related to available for sale securities.

+ References + Details

Balance Sheet Details (Tables)

12 Months Ended

Dec. 31, 2017

Balance Sheet Details [Abstract]

Summary of Property and Equipment

Property and equipment consist of the following (in thousands):

	December 31,
	2017			2016
Computer and office equipment	$	425		$	401
Scientific and laboratory equipment		5,494			3,965
Tenant improvements		1,706			1,687
		7,625			6,053
Less accumulated depreciation and amortization		(5,345	)		(4,632	)
	$	2,280		$	1,421

Summary of Accrued Expenses

Accrued expenses consist of the following (in thousands):

	December 31,
	2017		2016
Accrued salaries, wages and benefits		1,920		1,977
Other accrued expenses (1)		1,183		3,473
	$	3,103	$	5,450

(1)

Other accrued expenses include expenses for clinical research organizations and contract manufacturing organizations.

Debt, Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2017

Debt Instruments [Abstract]

Schedule of Term Loans and Unamortized Discount Balances

Term loans and unamortized discount balances are as follows (in thousands):

	December 31,
	2017			2016
Long-term debt	$	20,000		$	10,000
Less debt issuance costs and discount		(345	)		(463	)
Long-term debt, net of issuance costs and discount		19,655			9,537
Less current portion of long-term debt		(5,333	)		(339	)
Add accrual of final payment		397			—
Long-term debt, net of current portion and issuance costs and discount	$	14,719		$	9,198

Current portion of long-term debt	$	5,333		$	339
Less current portion of debt issuance costs and discount		(321	)		—
Current portion of long-term debt, net of issuance costs and discount	$	5,012		$	339

Schedule of Future Principal Payments for Term Loans

Future principal payments for the Term Loans are as follows (in thousands):

	December 31, 2017
2018	$	5,333
2019		8,000
2020		6,667
	$	20,000

Schedule of Future Minimum Payments under Non-cancelable Operating Lease

As of December 31, 2017, future minimum payments under the non-cancelable operating lease are as follows (in thousands):

	Operating Lease
2018	$	1,108
2019		420
	$	1,528

- Definition

Tabular disclosure of long-debt instruments or arrangements, including identification, terms, features, collateral requirements and other information necessary to a fair presentation. These are debt arrangements that originally required repayment more than twelve months after issuance or greater than the normal operating cycle of the entity, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21475-112644

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(e),(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 6: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21538-112644

Reference 7: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
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-Paragraph 6
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21506-112644

Reference 8: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
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-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399

Reference 9: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

Reference 10: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=84159872&loc=d3e21521-112644

+ Details

Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2017

Summary of Stock Option Activity

Stock option activity is summarized as follows:

	Number of Outstanding Options			Weighted Average Exercise Price		Weighted Remaining Contractual Term		Aggregate Intrinsic Value (in 000s)
Outstanding as of December 31, 2016		4,014,988		$	6.73
Granted		1,508,119		$	3.51
Exercised		(75,289	)	$	2.47
Canceled/forfeited/expired		(830,759	)	$	7.99
Outstanding as of December 31, 2017		4,617,059		$	5.52		7.37	$	1,370
Options vested and expected to vest as of December 31, 2017		4,617,059		$	5.52		7.37	$	1,370
Options exercisable as of December 31, 2017		2,220,670		$	6.01		6.23	$	1,070

Summary of Assumptions Used to Determine Fair Value of Employee Stock Purchase Plan

The assumptions used in the Black-Scholes option pricing model to determine the fair value of the ESPP offering were as follows:

	Years Ended December 31,
	2017			2016			2015
Expected term (in years)		0.50			0.50			0.50
Risk-free interest rate	0.6% – 1.0		%	0.4% – 0.6		%		0.3	%
Expected volatility	74.5% – 115.2		%	75.5% – 80.8		%		67.3	%
Expected dividend yield		0.0	%		0.0	%		0.0	%

Schedule of Restricted Stock Units Activity

During the year ended December 31, 2017, we granted restricted stock units to employees. Restricted stock unit activity is summarized as follows:

	Number of Outstanding Restricted Stock Units			Weighted Average Grant Date Fair Value
Balance as of December 31, 2016		76,713		$	4.95
Granted		22,000		$	3.30
Released		(36,099	)	$	4.96
Forfeited		(13,314	)	$	4.67
Balance as of December 31, 2017		49,300		$	4.28

Schedule of Allocation of Stock-Based Compensation for All Options Including Performance Options with Market Condition and Restricted Stock Units

The allocation of stock-based compensation for all options, including performance options with market condition and restricted stock units is as follows (in thousands):

	Years Ended December 31,
	2017		2016		2015
Research and development	$	1,399	$	1,876	$	2,524
General and administrative		5,385		3,153		2,332
	$	6,784	$	5,029	$	4,856

Summary of Warrants Outstanding

Warrants outstanding as of December 31, 2017 is as follows:

Number		Exercise Price		Expiration
Outstanding		Per Share		Date
	6,488,205	$	4.64	December 2019
	2,006	$	7.48	March 2021
	14,913	$	20.12	July 2023
	95,542	$	3.14	November 2023
	41,666	$	3.60	June 2024
	40,376	$	3.72	December 2024
	6,682,708

Summary of Common Stock Reserved for Future Issuance

Common stock reserved for future issuance is as follows:

	December 31,
	2017		2016
Class X Preferred Stock (if-converted to common stock)		11,429,760		—
Common stock warrants		6,682,708		121,512
Common stock options and awards outstanding		4,666,359		4,091,701
Shares available under the 2015 Plan		765,427		510,760
Shares available under the 2015 ESPP		583,819		407,542
		24,128,073		5,131,515

Employee Stock Option [Member]

Summary of Assumptions Used to Determine Fair Value of Employee Stock Option Grants and Performance Options with Market Condition

The assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants were as follows:

	Years Ended December 31,
	2017			2016			2015
Expected term (in years)	5.50 – 6.08			5.50 – 6.08			5.50 – 6.08
Risk-free interest rate	1.9% – 2.1		%	1.2% – 2.1		%	1.5% – 1.9		%
Expected volatility	99.1% – 124.4		%	80.7% – 84.5		%	79.2% – 100.9		%
Expected dividend yield		0.0	%		0.0	%		0.0	%

Performance Options with Market Condition [Member]

Summary of Assumptions Used to Determine Fair Value of Employee Stock Option Grants and Performance Options with Market Condition

The assumptions used at grant date to determine the fair value of the performance options with a market condition were as follows:

	December 31,
	2016			2015
Expected term (in years)		5.06			4.81
Risk-free interest rate		2.2	%		2.1	%
Expected volatility		83.3	%		80.6	%
Expected dividend yield		0.0	%		0.0	%

- Definition

Tabular disclosure of warrants or rights issued. Warrants and rights outstanding are derivative securities that give the holder the right to purchase securities (usually equity) from the issuer at a specific price within a certain time frame. Warrants are often included in a new debt issue to entice investors by a higher return potential. The main difference between warrants and call options is that warrants are issued and guaranteed by the company, whereas options are exchange instruments and are not issued by the company. Also, the lifetime of a warrant is often measured in years, while the lifetime of a typical option is measured in months. Disclose the title of issue of securities called for by warrants and rights outstanding, the aggregate amount of securities called for by warrants and rights outstanding, the date from which the warrants or rights are exercisable, and the price at which the warrant or right is exercisable.

+ References + Details

Income Tax (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Schedule of Pretax Earnings (Loss) for Domestic and Foreign Operations

Pretax earnings (loss) were generated by both domestic and foreign operations as follows (in thousands):

	Years Ended December 31,
	2017			2016			2015
United States	$	(47,712	)	$	(57,096	)	$	(47,490	)
Foreign		(495	)		(808	)		(483	)
	$	(48,207	)	$	(57,904	)	$	(47,973	)

Schedule of Reconciliations of the Expected Statutory Federal Income Tax

A reconciliation of the expected statutory federal income tax provision to the actual income tax provision is summarized as follows (in thousands):

	Years Ended December 31,
	2017			2016			2015
Expected income taxes benefit at federal statutory rate	$	(16,390	)	$	(19,687	)	$	(16,311	)
State income taxes, net of federal benefit		(13	)		—			—
Permanent items and other		1,311			675			865
Research credits		(2,286	)		(6,800	)		(2,674	)
Unrecognized tax benefits		914			2,720			1,070
Foreign rate differential		87			141			84
Change in tax rate		(25	)		—			3,551
Tax cuts and Jobs Act		27,933			—			—
Change in valuation allowance		(11,531	)		22,902			13,415
Income tax (benefit) expense	$	—		$	(49	)	$	—

Schedule of Significant Components of Deferred Tax Assets

Significant components of our deferred tax assets are summarized as follows (in thousands):

	December 31,
	2017			2016
Net operating loss carryforwards	$	27,226		$	33,713
Capitalized research and development expenses		16,218			21,624
Research credits and other state credits		11,229			9,227
Intangible assets		2,210			3,874
Reserve and accruals		2,843			2,711
Valuation allowance		(59,726	)		(71,149	)
Net deferred tax assets	$	—		$	—

Schedule of Activity Related to Unrecognized Tax Benefits

The activity related to our unrecognized tax benefits is summarized as follows (in thousands):

	December 31,
	2017			2016		2015
Balance as of beginning of year	$	13,000		$	5,033	$	1,106
Increase (decrease) related to prior year tax positions		(189	)		1,890		2,404
Increase related to current year tax positions		3,747			6,077		1,523
Balance as of end of year	$	16,558		$	13,000	$	5,033

Quarterly Financial Data (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2017

Quarterly Financial Data [Abstract]

Summary of Quarterly Financial Data

	For the quarters ended
	March 31			June 30			September 30			December 31
2017:
Operating expenses	$	13,211		$	11,907		$	10,827		$	11,200
Net loss		(13,405	)		(12,138	)		(11,190	)		(11,474	)
Basic and diluted net loss per share	$	(0.56	)	$	(0.51	)	$	(0.43	)		(0.39	)

2016:
Operating expenses	$	16,115		$	15,433		$	13,865		$	12,527
Net loss		(16,087	)		(15,383	)		(13,819	)		(12,566	)
Basic and diluted net loss per share	$	(0.68	)	$	(0.65	)	$	(0.58	)		(0.53	)

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Significant Accounting Policies [Line Items]
Impairment charges on investment securities	$ 0
Aggregate value of investment securities	64,000,000
Adjustment between amortized cost and fair value of investment securities	100,000
Impairment of long lived assets	$ 0
Weighted average shares subject to repurchase	3,685	25,984	61,814
Preferred stock redemption value	$ 0
Maximum [Member]
Significant Accounting Policies [Line Items]
Maturity period of investment securities	1 year
Estimated useful life of property and equipment	7 years
Minimum [Member]
Significant Accounting Policies [Line Items]
Estimated useful life of property and equipment	3 years
Percentage of tax benefit to be realized upon settlement	50.00%
Corporate Debt Securities [Member]
Significant Accounting Policies [Line Items]
Aggregate value of investment securities		$ 37,800,000
Adjustment between amortized cost and fair value of investment securities		$ 27,000
Corporate Debt Securities [Member] \| Maximum [Member]
Significant Accounting Policies [Line Items]
Maturity period of investment securities		1 year

Summary of Significant Accounting Policies - Schedule of Potentially Dilutive Securities Not Included in Calculation of Diluted Net Loss Per Share (Detail) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	22,809,913	4,250,049	2,668,613
Class X Convertible Preferred Stock (if-converted) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	11,429,760
Common Stock Options And Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	4,666,359	4,091,701	2,625,280
Warrants for Common Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	6,682,708	121,512	25,970
Employee Stock Purchase Plan [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities not included in calculation of diluted net loss per share	31,086	36,836	17,363

Summary of Significant Accounting Policies - Summary of Net Loss Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Consolidated net loss	$ (11,474)	$ (11,190)	$ (12,138)	$ (13,405)	$ (12,566)	$ (13,819)	$ (15,383)	$ (16,087)	$ (48,207)	$ (57,855)	$ (47,973)
Accretion to redemption value											(15)
Net loss attributable to common stockholders									$ (48,207)	$ (57,855)	$ (47,988)
Denominator:
Weighted average common shares outstanding									25,803,538	23,707,003	15,900,167
Weighted average common shares subject to repurchase									(3,685)	(25,984)	(61,814)
Weighted average common shares outstanding - basic and diluted									25,799,853	23,681,019	15,838,353
Net loss per share - basic and diluted	$ (0.39)	$ (0.43)	$ (0.51)	$ (0.56)	$ (0.53)	$ (0.58)	$ (0.65)	$ (0.68)	$ (1.87)	$ (2.44)	$ (3.03)

Fair Value Measurements - Schedule of Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 9,070	$ 29,251
Total assets measured at fair value	73,098	67,012
Available-for-sale [Member] \| Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	64,028	33,759
Available-for-sale [Member] \| Short-term Investments [Member] \| Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	21,943	7,843
Available-for-sale [Member] \| Short-term Investments [Member] \| Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	6,497
Available-for-sale [Member] \| Short-term Investments [Member] \| Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	18,260	20,913
Available-for-sale [Member] \| Short-term Investments [Member] \| United States Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	17,328	5,003
Available-for-sale [Member] \| Long-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		4,002
Available-for-sale [Member] \| Long-term Investments [Member] \| Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		4,002
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	9,070	29,251
Total assets measured at fair value	26,398	34,254
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Available-for-sale [Member] \| Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	17,328	5,003
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] \| Available-for-sale [Member] \| Short-term Investments [Member] \| United States Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	17,328	5,003
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	46,700	32,758
Significant Other Observable Inputs (Level 2) [Member] \| Available-for-sale [Member] \| Short-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	46,700	28,756
Significant Other Observable Inputs (Level 2) [Member] \| Available-for-sale [Member] \| Short-term Investments [Member] \| Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	21,943	7,843
Significant Other Observable Inputs (Level 2) [Member] \| Available-for-sale [Member] \| Short-term Investments [Member] \| Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	6,497
Significant Other Observable Inputs (Level 2) [Member] \| Available-for-sale [Member] \| Short-term Investments [Member] \| Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value	$ 18,260	20,913
Significant Other Observable Inputs (Level 2) [Member] \| Available-for-sale [Member] \| Long-term Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		4,002
Significant Other Observable Inputs (Level 2) [Member] \| Available-for-sale [Member] \| Long-term Investments [Member] \| Asset-backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, fair value		$ 4,002

Fair Value Measurements - Schedule of Available-for-sale Investments (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Short-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	$ 64,098	$ 33,787
Gross Unrealized Gains		1
Gross Unrealized Losses	(70)	(29)
Market Value	64,028	33,759
Short-term Investments [Member] \| Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	21,943	7,843
Market Value	21,943	7,843
Short-term Investments [Member] \| Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	18,286	20,942
Gross Unrealized Losses	(26)	(29)
Market Value	18,260	20,913
Short-term Investments [Member] \| United States Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	17,368	5,002
Gross Unrealized Gains		1
Gross Unrealized Losses	(40)
Market Value	17,328	5,003
Short-term Investments [Member] \| Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	6,501
Gross Unrealized Losses	(4)
Market Value	$ 6,497
Long-term Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost		4,001
Gross Unrealized Gains		1
Market Value		4,002
Long-term Investments [Member] \| Asset-backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost		4,001
Gross Unrealized Gains		1
Market Value		$ 4,002

Fair Value Measurements - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 Security
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments in gross unrealized loss position	20
Available-for-sale investments in gross unrealized loss position	12 months
Maximum [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments contractual maturity period	1 year

Balance Sheet Details - Summary of Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Line Items]
Property and equipment, Gross	$ 7,625	$ 6,053
Less accumulated depreciation and amortization	(5,345)	(4,632)
Property and equipment, Net	2,280	1,421
Computer and Office Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, Gross	425	401
Scientific and Laboratory Equipment [Member]
Property Plant And Equipment [Line Items]
Property and equipment, Gross	5,494	3,965
Tenant Improvements [Member]
Property Plant And Equipment [Line Items]
Property and equipment, Gross	$ 1,706	$ 1,687

Balance Sheet Details - Summary of Accrued Expenses (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Accrued Liabilities Current [Abstract]
Accrued salaries, wages and benefits	$ 1,920	$ 1,977
Other accrued expenses	1,183	3,473
Accrued expenses	$ 3,103	$ 5,450

Debt, Commitments and Contingencies - Additional Information (Detail)	1 Months Ended			7 Months Ended	12 Months Ended
	Apr. 30, 2017 USD ($) ft²	Jan. 31, 2017	Nov. 30, 2016 USD ($) Tranche $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 30, 2017 USD ($) $ / shares shares
Debt Instrument [Line Items]
Net cash proceeds from borrowings					$ 9,866,000	$ 9,736,000
Loan and security agreement, payment term					The first tranche of $10.0 million was funded in November 2016 (Term A Loan). Under the Term A Loan, we received cash proceeds of $7.3 million, net of a $2.6 million repayment of the principal, accrued interest and the $0.5 million final payment under our previous $10.0 million loan and security agreement with SVB (SVB Loan). We did not pay any termination or other fees in connection with the repayment of amounts due under the SVB Loan.
Extended term of lease		2 years
Noncancelable operating lease extension period	2019-05	2019-05
Additional lease facility \| ft²	7,411
Noncancelable operating lease total additional commitment	$ 700,000
Rent expense					$ 900,000	500,000	$ 400,000
Research and development expenses					30,067,000	42,846,000	34,504,000
Research Funding and Option Agreement [Member] \| National Foundation for Cancer Research [Member]
Debt Instrument [Line Items]
Charitable donations					200,000	400,000	400,000
Research Funding and Option Agreement [Member] \| The Scripps Research Institute [Member]
Debt Instrument [Line Items]
Research and development expenses					1,800,000	1,600,000	$ 700,000
Master Services Agreement [Member] \| ATYR1923 Manufacturer [Member]
Debt Instrument [Line Items]
Research and development expenses					$ 2,800,000	$ 1,100,000
Milestones payment terms					total payment in the low seven figures
Committed to pay based on development and production milestones				$ 1,600,000	$ 1,600,000
Silicon Valley Bank and Solar Capital, Ltd. [Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement			$ 20,000,000
Number of tranches \| Tranche			3
Term loan, debt issuance costs				900,000	900,000
Term loan, principal outstanding before deducting debt issuance cost				20,000,000	20,000,000
Final maturity payment accrued over life of term loan through interest expense				1,800,000	$ 1,800,000
Warrants expiration year					7 years
Aggregate fair value of warrants using black scholes option pricing model				$ 500,000	$ 500,000
Silicon Valley Bank and Solar Capital, Ltd. [Member] \| Term A Loan [Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement			$ 10,000,000
Net cash proceeds from borrowings			7,300,000
Repayment of debt, principal including accrued interest			2,600,000
Debt instrument, final payment			$ 500,000
Exercise price of warrant per share \| $ / shares			$ 3.14
Silicon Valley Bank and Solar Capital, Ltd. [Member] \| Term A Loan [Member] \| Common Stock [Member]
Debt Instrument [Line Items]
Warrants to purchase number of common stock, shares \| shares			47,771
Silicon Valley Bank and Solar Capital, Ltd. [Member] \| Term B Loan [Member]
Debt Instrument [Line Items]
Exercise price of warrant per share \| $ / shares								$ 3.60
Silicon Valley Bank and Solar Capital, Ltd. [Member] \| Term B Loan [Member] \| Common Stock [Member]
Debt Instrument [Line Items]
Warrants to purchase number of common stock, shares \| shares								20,833
Silicon Valley Bank and Solar Capital, Ltd. [Member] \| Term C Loan [Member]
Debt Instrument [Line Items]
Exercise price of warrant per share \| $ / shares				$ 3.72	$ 3.72
Silicon Valley Bank and Solar Capital, Ltd. [Member] \| Term C Loan [Member] \| Common Stock [Member]
Debt Instrument [Line Items]
Warrants to purchase number of common stock, shares \| shares				20,188	20,188
Silicon Valley Bank and Solar Capital, Ltd. [Member] \| Tranche One [Member]
Debt Instrument [Line Items]
Loan and security agreement funded date			Nov. 30, 2016
Silicon Valley Bank and Solar Capital, Ltd. [Member] \| Tranche One [Member] \| Term A Loan [Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement			$ 10,000,000
Silicon Valley Bank and Solar Capital, Ltd. [Member] \| Tranche Two [Member] \| Term B Loan [Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement			5,000,000
Silicon Valley Bank and Solar Capital, Ltd. [Member] \| Tranche Three [Member] \| Term C Loan [Member]
Debt Instrument [Line Items]
Available credit under loan and security agreement			$ 5,000,000
Loan Amendment Agreement with SVB and Solar [Member]
Debt Instrument [Line Items]
Loan and security agreement, payment term					Under the Loan Agreement, we were originally obligated to make interest only payments through December 1, 2017, followed by consecutive equal monthly payments of principal and interest in arrears through the maturity date of November 18, 2020. With the Loan Agreement, as amended, the interest only period was extended to June 1, 2018.
Loan amendment, description					In June 2017 and December 2017, we entered amendments to the Loan Agreement which modified certain conditions under which we may receive and repay the term loans under the Loan Agreement. The second (Term B) and third (Term C) tranche of $5.0 million each were funded in June 2017 and December 2017, respectively. We received total cash proceeds of $9.9 million, net of debt issuance costs of $0.1 million for both tranches. The amendments to the Loan Agreement were not a result of a troubled debt restructuring and did not result in modifications to the terms that were substantially different.
Maturity date					Nov. 18, 2020
Percentage of funded amounts for final payment			8.75%
Loan Amendment Agreement with SVB and Solar [Member] \| Prime Rate [Member]
Debt Instrument [Line Items]
Interest rate spread on variable rate			4.10%
Loan Amendment Agreement with SVB and Solar [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Prepayment fee percentage			1.00%
Loan Amendment Agreement with SVB and Solar [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Prepayment fee percentage			3.00%
Loan Amendment Agreement with SVB and Solar [Member] \| Tranche Two [Member] \| Term B Loan [Member]
Debt Instrument [Line Items]
Current borrowing capacity under loan and security agreement								$ 5,000,000
Loan Amendment Agreement with SVB and Solar [Member] \| Tranche Three [Member] \| Term C Loan [Member]
Debt Instrument [Line Items]
Current borrowing capacity under loan and security agreement				$ 5,000,000	$ 5,000,000
Loan Amendment Agreement with SVB and Solar [Member] \| Tranche Two and Three [Member]
Debt Instrument [Line Items]
Net cash proceeds from borrowings				9,900,000
Term loan, debt issuance costs				$ 100,000	$ 100,000

Debt, Commitments and Contingencies - Schedule of Term Loans and Unamortized Discount Balances (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Long-term debt, net of current portion and issuance costs and discount	$ 14,719	$ 9,198
Current portion of long-term debt, net of issuance costs and discount	5,012	339
Term Loans [Member]
Debt Instrument [Line Items]
Long-term debt	20,000	10,000
Less debt issuance costs and discount	(345)	(463)
Long-term debt, net of issuance costs and discount	19,655	9,537
Less current portion of long-term debt	(5,333)	(339)
Add accrual of final payment	397
Long-term debt, net of current portion and issuance costs and discount	14,719	9,198
Current portion of long-term debt	5,333	339
Less current portion of debt issuance costs and discount	(321)
Current portion of long-term debt, net of issuance costs and discount	$ 5,012	$ 339

Debt, Commitments and Contingencies - Schedule of Future Principal Payments for Term Loans (Detail) - Silicon Valley Bank and Solar Capital, Ltd. [Member] $ in Thousands	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
2018	$ 5,333
2019	8,000
2020	6,667
Long-term debt, net of issuance costs and discount	$ 20,000

Debt, Commitments and Contingencies - Schedule of Future Minimum Payments under Non-cancelable Operating Lease (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Leases Operating [Abstract]
2018	$ 1,108
2019	420
Operating Lease, total	$ 1,528

Stockholders' Equity - Additional Information (Detail)			1 Months Ended				3 Months Ended	12 Months Ended			15 Months Ended
Stockholders' Equity - Additional Information (Detail)	Aug. 31, 2017 USD ($) $ / shares	Aug. 27, 2017 Day $ / shares shares	Sep. 30, 2016 $ / shares shares	Jan. 31, 2016 $ / shares shares	Oct. 31, 2015 $ / shares shares	Apr. 30, 2015 shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Sep. 30, 2015	Jan. 01, 2018 shares	Jan. 01, 2017 shares	Jun. 30, 2016 USD ($)	Jan. 01, 2016 shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock, par value \| $ / shares							$ 0.001	$ 0.001	$ 0.001
Preferred stock, par value \| $ / shares							$ 0.001	$ 0.001	$ 0.001
Net proceeds from transaction, after giving effect to cost related to placement \| $								$ 42,477,000
Registration Statement filing date								2016-06
Number of common stock shares reserved for issuance							24,128,073	24,128,073	5,131,515
Stock options granted								1,508,119
Weighted average exercise price, granted \| $ / shares								$ 3.51
Weighted average grant date fair value of options granted \| $ / shares								$ 2.85	$ 3.34	$ 11.29
Total fair value of restricted stock units vested \| $								$ 100,000	$ 13,000	$ 0
Aggregate intrinsic value of stock options exercised \| $								300,000	34,000	1,900,000
Unrecognized share based compensation expense related to unvested stock options and restricted stock units \| $							$ 6,000,000	$ 6,000,000
Unrecognized cost expected to be recognized over a weighted average period								2 years 7 months 6 days
Stock-based compensation \| $								$ 6,784,000	$ 5,029,000	4,856,000
Chief Executive Officer [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock compensation expense \| $							$ 1,900,000
Chief Business Officer [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock compensation expense \| $								$ 300,000
2014 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum term for stock option plan grant								10 years
Options vesting period					4 years			4 years			6 years
Stock options granted					931,749
Stock-based compensation \| $										$ 800,000
2015 Stock Option and Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Maximum term for stock option plan grant						10 years
Options vesting period						4 years
Options vesting period, description								For an initial grant to an employee, 25% of the options generally vest on the first anniversary of the original vesting date, with the balance vesting monthly over the remaining three years. For subsequent grants to an employee, the options generally vest monthly over a four-year term.
2015 Stock Option and Incentive Plan [Member] \| One Year Anniversary [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Option vesting percentage						25.00%
2015 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of common stock shares reserved for issuance							583,819	583,819	407,542
Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Amount of offering, issuance and sale covered in base prospectus \| $														$ 150,000,000
Common Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued during the period										6,164,000
Common Stock [Member] \| Senior Vice President [Member] \| Inducement Pool Non-Qualified Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period			4 years
Stock options granted			145,000
Weighted average exercise price, granted \| $ / shares			$ 3.29
Common Stock [Member] \| Employees and Consultants [Member] \| Performance Options with Market Condition [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period					4 years
Stock options granted					169,402
Weighted average exercise price, granted \| $ / shares					$ 10.24
Achievement of specified market condition year and month					2017-10
Weighted average grant date fair value of options granted \| $ / shares					$ 4.23
Requisite service period					4 years 9 months 18 days
Common Stock [Member] \| Executives, Employees and Consultants [Member] \| Performance Options with Market Condition [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period				4 years
Stock options granted				396,960				0
Weighted average exercise price, granted \| $ / shares				$ 9.13
Achievement of specified market condition year and month								2018-01
Weighted average grant date fair value of options granted \| $ / shares				$ 1.93
Requisite service period				5 years 1 month 6 days
Common Stock [Member] \| One Year Anniversary [Member] \| Senior Vice President [Member] \| Inducement Pool Non-Qualified Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period			1 year
Option vesting percentage			25.00%
Common Stock [Member] \| Monthly Basis Over Three Years [Member] \| Senior Vice President [Member] \| Inducement Pool Non-Qualified Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options vesting period			3 years
Option vesting percentage			75.00%
Class X Convertible Preferred Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Preferred stock, shares issued							2,285,952	2,285,952	0
Cowen Company, LLC (Cowen) [Member] \| Sale Agreement [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Amount of offering, issuance and sale covered in sales agreement prospectus \| $														20,000,000
Agreed upon value of sale of common stock per transaction \| $														$ 35,000,000
IPO [Member] \| 2015 Stock Option and Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Effective date of plan								May 06, 2015
Number of common stock shares reserved for issuance						1,574,566
Stock option grants description								The number of shares reserved and available for issuance under the 2015 Plan will automatically increase each January 1, beginning on January 1, 2016 and thereafter until January 1, 2019, by the lesser of (i) 1,840,000 shares, (ii) 4% of the outstanding number of shares of our common stock on the immediately preceding December 31 or (iii) an amount determined by our board of directors.
Percentage threshold of outstanding shares as of December 31 of each year for calculation of annual increase in authorized shares under the plan						4.00%
Additional number of common stock shares reserved for issuance													949,793		946,803
IPO [Member] \| 2015 Stock Option and Incentive Plan [Member] \| Subsequent Event [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional number of common stock shares reserved for issuance												1,191,566
IPO [Member] \| 2015 Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Effective date of plan								May 06, 2015
Number of common stock shares reserved for issuance						227,623
Percentage threshold of outstanding shares as of December 31 of each year for calculation of annual increase in authorized shares under the plan						1.00%
Additional number of common stock shares reserved for issuance													237,448		236,700
IPO [Member] \| 2015 Employee Stock Purchase Plan [Member] \| Subsequent Event [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Additional number of common stock shares reserved for issuance												297,891
IPO [Member] \| Maximum [Member] \| 2015 Stock Option and Incentive Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Annual increase in shares authorized under plan, shares threshold						1,840,000
Securities Purchase Agreement [Member] \| Private Placement [Member] \| Common Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Fair value of common stock price per share \| $ / shares	$ 2.37
Preferred stock conversion price per share \| $ / shares	$ 2.37
Securities Purchase Agreement [Member] \| Private Placement [Member] \| Warrants [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Exercise price of warrant per share \| $ / shares		$ 4.64
Warrants expiration date		Dec. 31, 2019
Securities Purchase Agreement [Member] \| Private Placement [Member] \| VGO Fund [Member] \| Common Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued during the period		1,777,784
Shares issued, price per share \| $ / shares		$ 2.65
Common stock, par value \| $ / shares		$ 0.001
Securities Purchase Agreement [Member] \| Private Placement [Member] \| VGO Fund [Member] \| Warrants [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of number of shares purchased on as-converted basis		37.50%
Securities Purchase Agreement [Member] \| Private Placement [Member] \| VGO Fund [Member] \| Class X Convertible Preferred Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued, price per share \| $ / shares		$ 13.25
Preferred stock, shares issued		2,285,952
Preferred stock, par value \| $ / shares		$ 0.001
Convertible preferred stock, terms of conversion								Each share of Preferred Stock is convertible into five shares of our common stock. VGO Fund will be prohibited from converting the Preferred Stock into shares of our common stock if, as a result of such conversion, VGO Fund, together with its affiliates, would own more than 9.50% of the shares of our common stock then issued and outstanding, which percentage may change at VGO Fund’s election upon 61 days’ notice to us to (i) any other number less than or equal to 19.99% or (ii) subject to approval of our stockholders to the extent required in accordance with the NASDAQ Global Market rules, any number in excess of 19.99%.
Convertible preferred stock, common stock issued upon conversion		5
Maximum ownership percentage prohibiting conversion		9.50%
Number of days of notice \| Day		61
Maximum ownership percentage upon conversion, as per NASDAQ Global Market rules		19.99%
Securities Purchase Agreement [Member] \| Private Placement [Member] \| Remaining Purchasers [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Gross proceeds from transaction \| $	$ 45,800,000
Net proceeds from transaction, after giving effect to cost related to placement \| $	$ 42,500,000
Securities Purchase Agreement [Member] \| Private Placement [Member] \| Remaining Purchasers [Member] \| Common Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares issued during the period		4,094,336
Shares issued, price per share \| $ / shares		$ 2.65
Securities Purchase Agreement [Member] \| Private Placement [Member] \| Remaining Purchasers [Member] \| Warrants [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of number of shares purchased on as-converted basis		37.50%

Stockholders' Equity - Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Outstanding Options, Beginning Balance \| shares	4,014,988
Number of Outstanding Options, Granted \| shares	1,508,119
Number of Outstanding Options, Exercised \| shares	(75,289)
Number of Outstanding Options, Canceled/forfeited/expired \| shares	(830,759)
Number of Outstanding Options, Ending Balance \| shares	4,617,059
Number of Outstanding Options, Vested and Expected to Vest, Ending Balance \| shares	4,617,059
Number of Outstanding Options, Exercisable, Ending Balance \| shares	2,220,670
Weighted Average Exercise Price, Beginning Balance \| $ / shares	$ 6.73
Weighted Average Exercise Price, Granted \| $ / shares	3.51
Weighted Average Exercise Price, Exercised \| $ / shares	2.47
Weighted Average Exercise Price, Canceled/forfeited/expired \| $ / shares	7.99
Weighted Average Exercise Price, Ending Balance \| $ / shares	5.52
Weighted Average Exercise Price, Options Vested and Expected to Vest, Ending Balance \| $ / shares	5.52
Weighted Average Exercise Price, Options Exercisable, Ending Balance \| $ / shares	$ 6.01
Weighted Average Contractual Term, Outstanding	7 years 4 months 13 days
Weighted Average Contractual Term, Options Vested and Expected to Vest	7 years 4 months 13 days
Weighted Average Contractual Term, Options Exercisable	6 years 2 months 23 days
Aggregate Intrinsic Value, Outstanding \| $	$ 1,370
Aggregate Intrinsic Value, Options Vested and Expected to Vest \| $	1,370
Aggregate Intrinsic Value, Options Exercisable \| $	$ 1,070

Stockholders' Equity - Summary of Assumptions Used to Determine Fair Value of Employee Stock Option Grants, Employee Stock Purchase Plan and Performance Options with Market Condition (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
ESPP [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 months	6 months	6 months
Risk-free interest rate			0.30%
Risk-free interest rate, minimum	0.60%	0.40%
Risk-free interest rate, maximum	1.00%	0.60%
Expected volatility			67.30%
Expected volatility, minimum	74.50%	75.50%
Expected volatility, maximum	115.20%	80.80%
Expected dividend yield	0.00%	0.00%	0.00%
Employee Stock Option [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Risk-free interest rate, minimum	1.90%	1.20%	1.50%
Risk-free interest rate, maximum	2.10%	2.10%	1.90%
Expected volatility, minimum	99.10%	80.70%	79.20%
Expected volatility, maximum	124.40%	84.50%	100.90%
Expected dividend yield	0.00%	0.00%	0.00%
Employee Stock Option [Member] \| Minimum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	5 years 6 months	5 years 6 months	5 years 6 months
Employee Stock Option [Member] \| Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)	6 years 29 days	6 years 29 days	6 years 29 days
Performance Options with Market Condition [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term (in years)		5 years 22 days	4 years 9 months 22 days
Risk-free interest rate		2.20%	2.10%
Expected volatility		83.30%	80.60%
Expected dividend yield		0.00%	0.00%

Stockholders' Equity - Schedule of Restricted Stock Unit Activity (Detail) - Restricted Stock Unit [Member]	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Number of Outstanding Restricted Stock Units, Beginning Balance \| shares	76,713
Number of Outstanding Restricted Stock Units, Granted \| shares	22,000
Number of Outstanding Restricted Stock Units, Released \| shares	(36,099)
Number of Outstanding Restricted Stock Units, Forfeited \| shares	(13,314)
Number of Outstanding Restricted Stock Units, Ending Balance \| shares	49,300
Weighted Average Grant Date Fair Value, Beginning Balance \| $ / shares	$ 4.95
Weighted Average Grant Date Fair Value, Granted \| $ / shares	3.30
Weighted Average Grant Date Fair Value, Released \| $ / shares	4.96
Weighted Average Grant Date Fair Value, Forfeited \| $ / shares	4.67
Weighted Average Grant Date Fair Value, Ending Balance \| $ / shares	$ 4.28

Stockholders' Equity - Schedule of Allocation of Stock-Based Compensation for All Options Including Performance Options with Market Condition and Restricted Stock Units (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 6,784	$ 5,029	$ 4,856
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	1,399	1,876	2,524
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 5,385	$ 3,153	$ 2,332

Stockholders' Equity - Summary of Warrants Outstanding (Detail)	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Class Of Warrant Or Right [Line Items]
Number Outstanding	6,682,708
Warrant One [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	6,488,205
Exercise Price Per Share \| $ / shares	$ 4.64
Expiration Date	2019-12
Warrant Two [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	2,006
Exercise Price Per Share \| $ / shares	$ 7.48
Expiration Date	2021-03
Warrant Three [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	14,913
Exercise Price Per Share \| $ / shares	$ 20.12
Expiration Date	2023-07
Warrant Four [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	95,542
Exercise Price Per Share \| $ / shares	$ 3.14
Expiration Date	2023-11
Warrant Five [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	41,666
Exercise Price Per Share \| $ / shares	$ 3.60
Expiration Date	2024-06
Warrant Six [Member]
Class Of Warrant Or Right [Line Items]
Number Outstanding	40,376
Exercise Price Per Share \| $ / shares	$ 3.72
Expiration Date	2024-12

Stockholders' Equity - Summary of Common Stock Reserved for Future Issuance (Detail) - shares	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	24,128,073	5,131,515
Class X Preferred Stock (if-Converted to Common Stock) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	11,429,760
Common Stock Warrants [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	6,682,708	121,512
Common Stock Options and Awards [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	4,666,359	4,091,701
Shares Available Under the 2015 ESPP [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	583,819	407,542
Shares Available Under the 2015 Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock reserved for future issuance	765,427	510,760

Income Tax - Schedule of Pretax Earnings (Loss) for Domestic and Foreign Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
United States	$ (47,712)	$ (57,096)	$ (47,490)
Foreign	(495)	(808)	(483)
Loss before income taxes	$ (48,207)	$ (57,904)	$ (47,973)

Income Tax - Schedule of Reconciliations of the Expected Statutory Federal Income Tax (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Expected income taxes benefit at federal statutory rate	$ (16,390)	$ (19,687)	$ (16,311)
State income taxes, net of federal benefit	(13)
Permanent items and other	1,311	675	865
Research credits	(2,286)	(6,800)	(2,674)
Unrecognized tax benefits	914	2,720	1,070
Foreign rate differential	87	141	84
Change in tax rate	(25)		3,551
Tax cuts and Jobs Act	27,933
Change in valuation allowance	$ (11,531)	22,902	$ 13,415
Income tax (benefit) expense		$ (49)

Income Tax - Additional Information (Detail) - USD ($)	12 Months Ended
Income Tax - Additional Information (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax [Line Items]
Valuation allowance		$ 59,726,000	$ 71,149,000
Period of change in ownership		3 years
Percentage of change in ownership		50.00%
Unrecognized tax benefits, income tax penalties and interest accrued		$ 0	$ 0	$ 0
Change in unrecognized tax benefits		$ 0
U.S. federal corporate tax rate		34.00%
Remeasurement of deferred tax balance		$ 27,900,000
Scenario Forecast [Member]
Income Tax [Line Items]
U.S. federal corporate tax rate	21.00%
Federal [Member]
Income Tax [Line Items]
Net operating loss carryforwards		$ 112,500,000
Net operating loss carryforwards expiration year		2025
Research and development credit carryforward		$ 3,600,000
Research and development credit carryforward expiration year		2026
Federal [Member] \| Orphan Drug Credits [Member]
Income Tax [Line Items]
Research and development credit carryforward		$ 12,500,000
Research and development credit carryforward expiration year		2035
State [Member]
Income Tax [Line Items]
Net operating loss carryforwards		$ 119,200,000
Net operating loss carryforwards expiration year		2021
Research and development credit carryforward		$ 3,200,000
Foreign [Member]
Income Tax [Line Items]
Net operating loss carryforwards		$ 7,000,000

Income Tax - Schedule of Significant Components of Deferred Tax Assets (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 27,226	$ 33,713
Capitalized research and development expenses	16,218	21,624
Research credits and other state credits	11,229	9,227
Intangible assets	2,210	3,874
Reserve and accruals	2,843	2,711
Valuation allowance	$ (59,726)	$ (71,149)

Income Tax - Schedule of Activity Related to Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Balance as of beginning of year	$ 13,000	$ 5,033	$ 1,106
Increase (decrease) related to prior year tax positions	(189)	1,890	2,404
Increase related to current year tax positions	3,747	6,077	1,523
Balance as of end of year	$ 16,558	$ 13,000	$ 5,033

Employee Benefits - Additional Information (Detail) - 401 (k) Plan [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Plan Disclosure [Line Items]
Policy approved month and year	2015-04
Date of Policy beginning	Jun. 01, 2015
Percentage of match employee contribution	50.00%
Percentage of participants's annual salary	6.00%
Discretionary contribution	$ 0.2	$ 0.2	$ 0.1

Quarterly Financial Data (Unaudited) - Schedule of Quarterly Data for Interim Periods (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Operating expenses	$ 11,200	$ 10,827	$ 11,907	$ 13,211	$ 12,527	$ 13,865	$ 15,433	$ 16,115	$ 47,145	$ 57,940	$ 47,616
Net loss	$ (11,474)	$ (11,190)	$ (12,138)	$ (13,405)	$ (12,566)	$ (13,819)	$ (15,383)	$ (16,087)	$ (48,207)	$ (57,855)	$ (47,973)
Basic and diluted net loss per share	$ (0.39)	$ (0.43)	$ (0.51)	$ (0.56)	$ (0.53)	$ (0.58)	$ (0.65)	$ (0.68)	$ (1.87)	$ (2.44)	$ (3.03)

Subsequent Events - Additional Information (Detail) - Subsequent Event [Member] - USD ($) $ in Millions		1 Months Ended
	Jan. 04, 2018	Feb. 28, 2018
The Scripps Research Institute [Member] \| Research Funding and Option Agreement [Member]
Subsequent Event [Line Items]
Aggregate research funding amount		$ 2.0
Performance Options with Market Condition [Member]
Subsequent Event [Line Items]
Outstanding performance options forfeited	96,883

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	10. Subsequent Events On January 4, 2018, the market condition for the performance options with market conditions granted in January 2016 was not met. As a result, 96,883 performance options, the outstanding performance options as of January 4, 2018, were forfeited. In February 2018, we amended the agreement with TSRI to provide additional funding bringing the total to $2.0 million for the preceding 12 months.

Organization, Business and Basis of Presentation - Additional Information (Detail)	Dec. 31, 2017
Pangu BioPharma [Member] \| Hong Kong [Member]
Description Of Business [Line Items]
Majority-owned subsidiary percentage	98.00%