Global Blood Therapeutics (GBT) 10-K 27 Feb 19 2018 FY Annual report Financial data

Document And Entity Information

Document And Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2018	Feb. 22, 2019	Jun. 30, 2018
Document Information [Line Items]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Entity Registrant Name	Global Blood Therapeutics, Inc.
Entity Central Index Key	1,629,137
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	Large Accelerated Filer
Entity Public Float			$ 2,286,912,803
Trading Symbol	GBT
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		56,322,257
Entity Emerging Growth Company	false
Entity Small Business	false

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 275,357	$ 198,332
Short-term marketable securities	202,177	116,493
Prepaid expenses	6,337	3,839
Other assets, current	1,909	5,648
Total current assets	485,780	324,312
Property and equipment, net	14,981	16,571
Long-term marketable securities	114,281	14,607
Restricted cash	2,395	1,046
Other assets, noncurrent	206	184
Total assets	617,643	356,720
Current liabilities:
Accounts payable	6,046	7,177
Accrued liabilities	16,792	10,135
Accrued compensation	10,036	8,579
Other liabilities, current	899	373
Total current liabilities	33,773	26,264
Other liabilities, noncurrent	11,071	11,652
Total liabilities	44,844	37,916
Commitments and contingencies (Note 10)
Stockholders' equity:
Preferred stock, $0.001 par value, 5,000,000 shares authorized at December 31, 2018 and 2017, respectively, and none issued and outstanding as of December 31, 2018 and 2017.
Common stock, $0.001 par value, 150,000,000 shares authorized at December 31, 2018 and 2017, respectively; 55,640,299 and 46,131,723 shares issued and outstanding at December 31, 2018 and 2017, respectively.	56	46
Additional paid-in capital	1,044,941	617,051
Accumulated other comprehensive loss	(48)	(336)
Accumulated deficit	(472,150)	(297,957)
Total stockholders' equity	572,799	318,804
Total liabilities and stockholders' equity	$ 617,643	$ 356,720

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares issued	55,640,299	46,131,723
Common stock, shares outstanding	55,640,299	46,131,723

Consolidated Statements of Oper

Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating expenses:
Research and development	$ 131,307	$ 87,807	$ 62,163
General and administrative	51,435	31,438	20,964
Total operating expenses	182,742	119,245	83,127
Loss from operations	(182,742)	(119,245)	(83,127)
Interest income, net	8,618	2,555	659
Other expenses, net	(69)	(334)
Net loss	(174,193)	(117,024)	(82,468)
Other comprehensive loss:
Net unrealized gain (loss) on marketable securities, net of tax	288	(170)	(166)
Comprehensive loss	$ (173,905)	$ (117,194)	$ (82,634)
Basic and diluted net loss per common share	$ (3.41)	$ (2.76)	$ (2.48)
Weighted-average number of shares used in computing basic and diluted net loss per common share	51,150,728	42,323,686	33,207,382

Consolidated Statements of Rede

Consolidated Statements of Redeemable Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Loss [Member]	Accumulated Deficit [Member]
Beginning Balance at Dec. 31, 2015	$ 140,795	$ 29	$ 239,231		$ (98,465)
Beginning Balance, Shares at Dec. 31, 2015		29,359,800
Issuance of common stock upon equity offering, net of issuance costs	116,995	$ 7	116,988
Issuance of common stock upon equity offering, net of issuance costs, Shares		6,667,228
Issuance of common stock upon exercise of stock options	222		222
Common stock issued on exercise of stock options, Shares		147,126
Issuance of common stock pursuant to ESPP purchases	1,018		1,018
Issuance of common stock pursuant to ESPP purchases, Shares		65,252
Vesting of restricted stock purchases	678	$ 1	677
Vesting of restricted stock purchases, Shares		398,750
Stock-based compensation expense	9,235		9,235
Net unrealized gain (loss) on marketable securities	(166)			$ (166)
Net loss	(82,468)				(82,468)
Ending Balance at Dec. 31, 2016	186,309	$ 37	367,371	(166)	(180,933)
Ending Balance, Shares at Dec. 31, 2016		36,638,156
Issuance of common stock upon equity offering, net of issuance costs	231,955	$ 8	231,947
Issuance of common stock upon equity offering, net of issuance costs, Shares		8,498,926
Issuance of common stock upon exercise of stock options	2,816	$ 1	2,815
Common stock issued on exercise of stock options, Shares		578,455
Issuance of common stock upon vesting of restricted share units, net of shares withheld for employee taxes	(238)		(238)
Issuance of common stock upon vesting of restricted share units, net of shares withheld for employee taxes, Shares		33,212
Issuance of common stock pursuant to ESPP purchases	1,050		1,050
Issuance of common stock pursuant to ESPP purchases, Shares		76,585
Vesting of restricted stock purchases	424		424
Vesting of restricted stock purchases, Shares		306,389
Stock-based compensation expense	13,682		13,682
Net unrealized gain (loss) on marketable securities	(170)			(170)
Net loss	(117,024)				(117,024)
Ending Balance at Dec. 31, 2017	318,804	$ 46	617,051	(336)	(297,957)
Ending Balance, Shares at Dec. 31, 2017		46,131,723
Issuance of common stock upon equity offering, net of issuance costs	396,034	$ 8	396,026
Issuance of common stock upon equity offering, net of issuance costs, Shares		8,403,826
Issuance of common stock upon exercise of stock options	$ 6,022	$ 1	6,021
Common stock issued on exercise of stock options, Shares	596,434	596,434
Issuance of common stock upon vesting of restricted share units, net of shares withheld for employee taxes	$ (6,252)	$ 1	(6,253)
Issuance of common stock upon vesting of restricted share units, net of shares withheld for employee taxes, Shares		255,039
Issuance of common stock pursuant to ESPP purchases	1,647		1,647
Issuance of common stock pursuant to ESPP purchases, Shares		61,031
Vesting of restricted stock purchases	369		369
Vesting of restricted stock purchases, Shares		192,246
Stock-based compensation expense	30,080		30,080
Net unrealized gain (loss) on marketable securities	288			288
Net loss	(174,193)				(174,193)
Ending Balance at Dec. 31, 2018	$ 572,799	$ 56	$ 1,044,941	$ (48)	$ (472,150)
Ending Balance, Shares at Dec. 31, 2018		55,640,299

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (174,193)	$ (117,024)	$ (82,468)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	4,607	1,658	1,160
Amortization (accretion) of premium (discount) on marketable securities	(661)	704	75
Stock-based compensation	30,080	13,682	9,235
Loss from disposal of fixed assets, net	45
Changes in operating assets and liabilities:
Prepaid expenses	(2,500)	(1,856)	(761)
Other assets, current	(1,278)	(162)	(512)
Accounts payable	(1,285)	2,771	1,066
Accrued liabilities	8,031	3,280	1,794
Accrued compensation	1,457	3,612	2,725
Other liabilities, current	742	(63)	23
Other liabilities, noncurrent	(420)	376	(60)
Net cash used in operating activities	(135,375)	(93,022)	(67,723)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(4,824)	(3,101)	(1,352)
Sale of property and equipment	75
Purchases of marketable securities	(361,405)	(127,724)	(105,500)
Maturities of marketable securities	181,997	96,000
Purchases of other assets		(175)
Net cash used in investing activities	(184,157)	(35,000)	(106,852)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock, net of issuance costs	396,501	231,955	116,995
Proceeds from issuance of common stock in settlement of employee stock purchase plan and exercise of stock options	7,669	3,892	1,240
Repurchases of unvested restricted stock purchases	(12)	(421)	(90)
Taxes paid related to net share settlement of equity awards	(6,252)	(238)
Net cash provided by financing activities	397,906	235,188	118,145
Net increase (decrease) in cash, cash equivalents and restricted cash	78,374	107,166	(56,430)
Cash, cash equivalents and restricted cash at beginning of period	199,378	92,212	148,642
Cash,cash equivalents and restricted cash at end of period	277,752	199,378	92,212
SUPPLEMENTAL DISCLOSURES OF NON-CASH FINANCING INFORMATION:
Accrued issuance costs	467
Leasehold improvements paid for by landlord		11,086
Accrued purchase of property and equipment	$ 48	$ 1,536	$ 114

Organization and Basis of Prese

Organization and Basis of Presentation	12 Months Ended
	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Organization and Basis of Presentation	1. Organization and Basis of Presentation Global Blood Therapeutics Inc. (the “Company”, “we”, “us”, and “our”) was incorporated in Delaware in February 2011 and commenced operations in May 2012. We are a clinical-stage biopharmaceutical company determined to discover, develop and deliver innovative treatments that provide hope to underserved patient communities. Our primary activities have been establishing our facilities, recruiting personnel, conducting development of our product candidates, including clinical trials, and raising capital. Our principal operations are based in South San Francisco, California, and we operate in one segment. Follow-on In December 2017, we completed a follow-on In March 2018, we completed a follow-on In December 2018, we c ompleted a follow-on Need for Additional Capital In the course of our development activities, we have sustained operating losses and we expect such losses to continue over the next several years. Our ultimate success depends on the outcome of our research and development activities. As of December 31 , 2018, we had an accumulated deficit of $ 472.2 million. We expect to incur additional losses in the future to conduct product research and development and we anticipate the need to raise additional capital to fully implement our business plan. We intend to raise such capital through the issuance of additional equity, potentially through borrowings, and strategic alliances with partner companies. However, if such financing is not available at adequate levels or when it will be required, we will need to reevaluate our operating plans. We believe that our existing capital resources consisting of cash and cash equivalent and marketable securities will be sufficient to fund our operations for at least the next twelve months from the date of issuance of these financial statements.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”). Use of Estimates The preparation of the accompanying consolidated financial statements in accordance with U.S. GAAP requires us to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of costs and expenses during the reporting period. We base our estimates and assumptions on historical experience when available and on various factors that we believe to be reasonable under the circumstances. We evaluate our estimates and assumptions on an ongoing basis. Our actual results could differ from these estimates under different assumptions or conditions. Principles of Consolidation The accompanying consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All significant intercompany transactions and balances have been eliminated upon consolidation. Segment Reporting We have determined that we operate in a single segment based upon the way the business is organized for making operating decisions and assessing performance. The Company has only one operating segment related to the development of pharmaceutical products. All property and equipment is maintained in the United States. Cash and Cash Equivalents We consider all highly liquid investments with original maturities of three months or less at the time of purchase to be cash equivalents. Cash equivalents, which consist primarily of amounts invested in money market accounts, are stated at fair value. Concentration of Credit Risk We invest in a variety of financial instruments and, by our policy, limit the amount of credit exposure with any one issuer, industry or geographic area for investments other than instruments backed by the U.S. federal government. Investments in Marketable Securities We invest in marketable securities, primarily money market funds, corporate debt securities, government securities, government agency securities, and certificates of deposits. We classify our marketable securities as available-for-sale available-for-sale available-for-sale We regularly review all of our investments for other-than-temporary declines in estimated fair value. Our review includes the consideration of the cause of the impairment, including the creditworthiness of the security issuers, the number of securities in an unrealized loss position, the severity and duration of the unrealized losses, whether we have the intent to sell the securities and whether it is more likely than not that we will be required to sell the securities before the recovery of their amortized cost basis. When we determine that the decline in estimated fair value of an investment is below the amortized cost basis and the decline is other-than-temporary, we reduce the carrying value of the security and record a loss for the amount of such decline. Fair Value Measurement The carrying amounts of certain financial instruments, including cash and cash equivalents, other receivables as included in other assets, current, restricted cash, accounts payable and accrued liabilities approximate fair value due to their relatively short maturities. Property and Equipment, Net Property and equipment are stated at cost, less accumulated depreciation and amortization. Depreciation is provided using the straight-line method over the estimated useful lives of the assets, three years for computer equipment and five years for laboratory equipment. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the improvements. Depreciation and amortization begins at the time the asset is placed in service. Maintenance and repairs are charged as expense in the statements of operations and comprehensive loss as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation and amortization are removed from the consolidated balance sheet and any resulting gain or loss is reflected in operations. Impairment of Long-Lived Assets We evaluate our long-lived assets, including property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying value of these assets may not be recoverable. Recoverability of these assets is measured by comparison of the carrying amount of each asset to the future undiscounted cash flows the asset is expected to generate over its remaining life. If the asset is considered to be impaired, the amount of any impairment is measured as the difference between the carrying value and the fair value of the impaired asset. There have been no impairments of our long-lived assets for the periods presented. Restricted Cash Restricted cash consists of cash deposits held by our financial institution as collateral for our letter of credit under our facility lease. Accruals of Research and Development Costs We record accruals for estimated costs of research, nonclinical and clinical studies and manufacturing development. These costs are a significant component of our research and development expenses. A substantial portion of our ongoing research and development activities are conducted by third-party service providers, including contract research organizations and contract manufacturing organizations. We accrue the costs incurred under our agreements with these third parties based on actual work completed in accordance with agreements established with these third parties. We determine the actual costs through discussions with internal personnel and external service providers as to the progress or stage of completion of the services and the agreed-upon fee to be paid for such services. We make judgments and estimates in determining the accrual balance in each reporting period. As actual costs become known, we adjust our accruals. We have not experienced any material deviations between accrued clinical trial expenses and actual clinical trial expenses. However, actual services performed, number of subjects enrolled, and the rate of subject enrollment may vary from our estimates, resulting in adjustments to clinical trial expense in future periods. Changes in these estimates that result in material changes to our accruals could materially affect our results of operations. Leases We enter into lease agreements for our office and laboratory facilities. These leases are classified as operating leases. Rent expense is recognized on a straight-line basis over the noncancelable term of the lease and, accordingly, we record the difference between cash rent payments and the recognition of rent expense as a deferred rent liability, which is included within other liabilities on the consolidated balance sheet. Incentives granted under our facilities leases, including rent holiday and allowances to fund leasehold improvements, are deferred and are recognized as adjustments to rental expense on a straight-line basis over the noncancelable term of the lease. Comprehensive Income (Loss) Comprehensive income (loss) is defined as a change in equity of a business enterprise during a period, resulting from transactions from non-owner Research and Development Research and development costs are expensed as incurred and consist of salaries and benefits, stock-based compensation expense, lab supplies and facility costs, as well as fees paid to other nonemployees and entities that conduct certain research and development activities on our behalf. Amounts incurred in connection with license agreements are also included in research and development expense. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are capitalized and then expensed as the related goods are delivered or the services are performed. Stock-Based Compensation We measure and recognize stock-based compensation expense, including employee and non-employee Income Taxes We use the asset and liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. We must then assess the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to our lack of earnings history, the net deferred tax assets have been fully offset by a valuation allowance. We recognize benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on their technical merits, as the largest amount of benefit that is more likely than not to be realized upon the ultimate settlement. It is our policy to include penalties and interest expense related to income taxes as a component of other expense and interest expense, respectively, as necessary. To date, there have been no interest or penalties incurred in relation to the unrecognized tax benefits. Net Loss per Share Basic net loss per share is calculated by dividing the net loss by the weighted average number of shares of common stock outstanding during the period, without consideration of common stock equivalents. Diluted net loss per share is the same as basic net loss per share, since the effects of potentially dilutive securities are antidilutive given our net loss. Recent Accounting Pronouncements Leases (Topic 842) In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) right-of-use During 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842) – Targeted Improvements The new standard also provides a number of optional practical expedients for the transition from ASC 840 – Leases (Topic 840) to Topic 842 that allow entities to not (i) reassess whether any expired or existing contracts are considered or contain leases; (ii) reassess the lease classification for any expired or existing leases; and (iii) reassess initial direct costs for any existing leases. These practical expedients, if elected, must be elected as a package and applied consistently by an entity to all of its leases. We plan to elect the use of practical expedients as a package. We expect that this standard will have a material effect on our consolidated financial statements. While we continue to evaluate the provisions of ASC 842 to determine the impact the adoption will have on our consolidated financial statements, we currently believe the most significant effects relate to the recognition of new right-of-use (ROU) assets and lease liabilities on our consolidated balance sheet related to our office and equipment operating leases. We do not expect a significant change in our leasing activities between now and adoption. On adoption, we currently expect to recognize additional lease liabilities of approximately $25.9 million and corresponding ROU assets of approximately $14.2 million, which reflect lease incentives previously received of approximately $11.7 The new standard also provides practical expedients for an entity’s ongoing accounting. We currently expect to elect the short-term lease recognition exemption for all leases that qualify. This means, for those leases that qualify, we will not recognize ROU assets or lease liabilities, and this includes not recognizing ROU assets or lease liabilities for existing short-term leases of those assets in transition. We also currently expect to elect the practical expedient to not separate lease and non-lease components for all of our leases. Other recent accounting pronouncements In February 2018, the FASB issued ASU No. 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820) In August 2018, the FASB issued ASU No. 2018-15, Intangibles – Goodwill and Other – Internal-Use Software (Subtopic 350-40), Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract Accounting Pronouncements Adopted In August 2016, the FASB issued ASU No. 2016-15, Classification of Certain Cash Receipts and Cash Payments In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash In May 2017, the FASB issued ASU No. 2017-09, Compensation – Stock Compensation (Topic 718) In March 2018, the FASB issued ASU No. 2018-04, Investments – Debt Securities (Topic 320) and Regulated Operations (Topic 980): Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 117 and SEC Release No. 33-9273. Investments-Debt Securities Regulated Operations In March 2018, the FASB issued ASU No. 2018-05, Income Taxes (Topic 740): Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118 Income Taxes In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	3. Fair Value Measurements Fair value accounting is applied for all financial assets and liabilities that are recognized or disclosed at fair value in the consolidated financial statements on a recurring basis (at least annually). Our financial instruments consist of cash and cash equivalents, marketable securities, other receivables as included in other assets, current, restricted cash, accounts payable and accrued liabilities. Cash and cash equivalents, marketable securities and restricted cash are reported at their respective fair values on our Consolidated Balance Sheets. The remaining financial instruments are reported on our Consolidated Balance Sheets at cost that approximate current fair values due to their relatively short maturities. Assets and liabilities recorded at fair value on a recurring basis in the consolidated balance sheets are categorized based upon the level of judgment associated with the inputs used to measure their fair values. Fair value is defined as the exchange price that would be received for an asset or an exit price that would be paid to transfer a liability in the principal or most advantageous market for the asset or liability in an orderly transaction between market participants on the measurement date. The authoritative guidance on fair value measurements establishes a three-tier fair value hierarchy for disclosure of fair value measurements as follows: Level 1— Level 2— Level 3— The following table summarizes our financial assets measured at fair value on a recurring basis (in thousands): December 31, 2018 Total Level 1 Level 2 Level 3 Financial Assets: Money market funds $ 275,234 $ 275,234 $ — $ — Corporate debt securities 110,027 — 110,027 — U.S. government agency securities 88,028 — 88,028 — Certificates of deposits 6,675 — 6,675 — U.S. government securities 111,728 — 111,728 — Total financial assets $ 591,692 $ 275,234 $ 316,458 $ — December 31, 2017 Total Level 1 Level 2 Level 3 Financial Assets: Money market funds $ 185,824 $ 185,824 1 $ — $ — Corporate debt securities 46,977 — 46,977 — U.S. government agency securities 54,989 — 54,989 — Certificates of deposits 9,129 — 9,129 — U.S. government securities 20,007 — 20,007 — Total financial assets $ 316,926 $ 185,824 $ 131,102 $ — (1) In 2017, some balances have been reclassified from cash to money market funds in the above table. We estimate the fair values of our investments in corporate debt securities, government and government related securities and certificates of deposits by taking into consideration valuations obtained from third-party pricing services. The fair value of our marketable securities classified within Level 2 is based upon observable inputs that may include benchmark yields, reported trades, broker/dealer quotes, issuer spreads, two-sided A-1/P-1/F1

Available-for-Sale Securities

Available-for-Sale Securities	12 Months Ended
	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale Securities	4. Available-for-Sale Estimated fair values of available-for-sale The following table is a summary of available-for-sale December 31, 2018 December 31, 2017 Amortized Cost Unrealized Gains Unrealized (Losses) Estimated Fair Value Amortized Cost Unrealized Gains Unrealized (Losses) Estimated Fair Value Financial Assets: Money market funds $ 275,234 $ — $ — $ 275,234 $ 185,824 $ — $ — $ 185,824 1 Corporate debt securities 110,053 69 (95 ) 110,027 47,108 — (131 ) 46,977 U.S. government agency securities 88,042 40 (54 ) 88,028 55,170 — (181 ) 54,989 Certificates of deposits 6,681 1 (7 ) 6,675 9,142 — (13 ) 9,129 U.S. government securities 111,730 60 (62 ) 111,728 20,018 — (11 ) 20,007 Total $ 591,740 $ 170 $ (218 ) $ 591,692 $ 317,262 $ — $ (336 ) $ 316,926 (1) In 2017, some balances have been reclassified from cash to money market funds in the above table. The following table summarizes the classification of the available-for-sale securities on our Consolidated Balance Sheets (in thousands): December 31, 2018 December 31, 2017 Cash and cash equivalents $ 275,234 $ 185,826 1 Short-term marketable securities 202,177 116,493 Long-term marketable securities 114,281 14,607 Total $ 591,692 $ 316,926 (1) In 2017, some balances have been reclassified from cash to money market funds in the above table. We do not intend to sell the investments that are in an unrealized loss position, and it is unlikely that we will be required to sell the investments before recovery of their amortized cost basis, which may be maturity. We have determined that the gross unrealized losses on our marketable securities at December 31, 2018 were temporary in nature. All unrealized losses from all marketable securities at December 31, 2018 are not material.

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Balance Sheet Components	5. Balance Sheet Components Property and Equipment Property and equipment consist of the following (in thousands): December 31, 2018 2017 Laboratory equipment $ 7,363 $ 5,715 Computer equipment 1,501 1,594 Leasehold improvements 13,785 12,642 Construction-in-progress 239 419 Total property and equipment 22,888 20,370 Less: accumulated depreciation and amortization (7,907 ) (3,799 ) Property and equipment, net $ 14,981 $ 16,571 Depreciation expense was $4.7 million for the year ended December 31, 2018, $1.7 million for the year ended December 31, 2017 and $1.2 million for the year ended December 31, 2016. Refer to Note 10 – Commitments and Contingencies for details on acceleration of depreciation expenses recognized during the year ended December 31, 2018. Accrued Liabilities Accrued liabilities consist of the following (in thousands): December 31, 2018 2017 Accrued clinical and manufacturing expenses $ 15,121 $ 8,035 Accrued professional and consulting services 1,016 1,007 Other 655 1,093 Total accrued liabilities $ 16,792 $ 10,135 Other liabilities, current and noncurrent Other liabilities consist of the following (in thousands): December 31, 2018 2017 Restricted shares subject to repurchase, current $ 157 $ 373 Deferred rent, current 712 — Other payable, current 30 — Total other liabilities, current $ 899 $ 373 Restricted shares subject to repurchase, noncurrent $ — $ 161 Deferred rent, noncurrent 11,041 11,491 Other payable, noncurrent 30 — Total other liabilities, noncurrent $ 11,071 $ 11,652

Stockholders' Equity

Stockholders' Equity	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Stockholders' Equity	6. Stockholders’ Equity Common Stock Reserved for Issuance We have reserved sufficient shares of common stock for issuance upon the exercise of stock options, vesting of restricted stock units and restricted shares subject to future vesting. Common stockholders are entitled to dividends if and when declared by the board of directors, subject to the prior rights of any preferred stockholders. As of December 31, 2018, no common stock dividends had been declared by the board of directors. We have reserved shares of common stock, on an as-converted December 31, 2018 2017 Restricted shares subject to future vesting 47,051 241,617 Restricted stock units 975,419 820,713 Options issued and outstanding 3,243,551 2,945,901 Shares available for future grant under the 2015 Plan and 2017 Inducement Equity Plan 3,003,454 1,708,680 Employee stock purchase plan 240,935 186,033 Total 7,510,410 5,902,944 Restricted Stock We have issued restricted stock awards to employees under our 2012 Stock Option and Grant Plan (the “2012 Plan”). Under the related stock purchase agreements, we have the right to repurchase the common stock at the lower of fair market value and the stockholders’ original purchase price which right lapses according to individual vesting schedules. In order to vest, the holders are required to provide continued service to us. Upon vesting, the appropriate amounts are transferred from liabilities to additional paid-in Restricted shares subject to repurchase were as follows: December 31, 2018 2017 Restricted shares subject to repurchase: Shares issued pursuant to the 2012 Stock Option and Grant Plan 47,051 241,617 Total restricted shares subject to repurchase 47,051 241,617

Share-based Compensation

Share-based Compensation	12 Months Ended
	Dec. 31, 2018
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Share-based Compensation	7. Share-based Compensation 2017 Inducement Equity Plan In January 2017, we adopted the 2017 Inducement Equity Plan (the “ 2017 Inducement Plan”). Under the 2017 Inducement Plan, shares of our common stock are reserved for the issuance of non-qualified 31 , 2018 , there were 458,350 shares reserved for the future issuance of equity awards under the 2017 Inducement Plan. 2015 Stock Option and Incentive Plan In July 2015, we adopted the 2015 Stock Option and Incentive Plan (the “2015 Plan”). Under the 2015 Plan, shares of our common stock are reserved for the issuance of stock options, restricted stock, and other equity-based awards to employees, non-employee non-statutory 4-year 2012 Stock Option and Grant Plan In 2012, the Company adopted the 2012 Stock Option and Grant Plan (the “2012 Plan”) under which our Board of Directors was authorized to grant incentive stock options to employees, including officers and members of the Board of Directors who are also employees of ours, and non-statutory 3,785,713 Stock Option Activity The following table summarizes activity under the Company’s stock option plans, including the 2017 Inducement Plan, 2015 Plan and the 2012 Plan and related information (in thousands, except share and per share amounts and term): Number Weighted- Weighted- Aggregate Outstanding — December 31, 2017 2,945,901 $ 17.50 8.25 Options granted 1,102,208 52.80 Options exercised (596,434 ) 10.10 Options canceled (208,124 ) 34.84 Outstanding — December 31, 2018 3,243,551 $ 29.74 7.96 $ 49,373 Vested and exercisable — December 31, 2018 1,553,778 $ 21.80 7.31 $ 32,305 The aggregate intrinsic value was calculated as the difference between the exercise price of the options and the fair value of our common stock as of December 31, 2018. The total intrinsic value of options exercised was $23.4 million for the year ended December 31, 2018, $11.3 million for the year ended December 31 , 2017 and $2.4 million for the year ended December 31 , 2016 . The weighted-average estimated fair value of stock options granted was $33.58 for the year ended December 31 , 2018 , $16.19 for the year ended December 31 , 2017 and $11.74 for the year ended December 31 , 2016 . Stock Options Granted to Employees with Service-based Vesting Valuation Assumptions The fair values of stock options granted to employees were calculated using the following assumptions: Year Ended December 31, 2018 2017 2016 Expected term (in years) 5.3-6.1 5.3-6.1 5.3-6.1 Volatility 68.7%-71.8% 68.9%-75.6% 70.6%-82.3% Risk-free interest rate 2.6%-3.0% 1.8%-2.3% 1.1%-2.1% Dividend yield — — — In determining the fair value of the options granted, we used the Black-Scholes-Merton option-pricing model and assumptions discussed below. Expected Term mid-point Expected Volatility Risk-Free Interest Rate Expected Dividend Restricted Stock Units In January 2017, the Compensation Committee of our Board of Directors approved the commencement of granting restricted stock units (“RSUs”) to our employees. RSUs are share awards that entitle the holder to receive freely tradable shares of our common stock upon the completion of a specific period of continued service. RSUs are generally subject to forfeiture if employment terminates prior to the release of vesting restrictions. RSUs granted are valued at the market price of our common stock on the date of grant. We recognize noncash compensation expense for the fair value of RSUs on a straight-line basis over the requisite service period of these awards. The following table summarizes activity of RSUs granted to employees with service-based vesting under the 2017 Inducement Plan and 2015 Plan and related information (in thousands, except share, per share amounts and vesting period): Number Weighted- Weighted- Aggregate Non-vested 467,463 $ 24.93 1.71 $ 18,395 RSUs granted 625,765 54.48 RSUs vested (179,772 ) 34.38 RSUs forfeited (97,287 ) 41.89 Non-vested 816,169 $ 43.34 1.54 $ 33,504 Restricted Stock Purchases When Restricted Stock Purchases (“RSPs”) are granted, the individual purchases the shares at the grant date fair value of the underlying common stock. The purchase of the stock is subject to forfeiture prior to vesting at the lower of fair value and the original purchase price. The award is treated similarly to an early exercise of stock options for accounting purposes. A summary of our unvested restricted stock for the year ended December 31, 2018 is as follows: Number of RSPs Weighted Average Outstanding — December 31, 2017 241,617 $ 1.46 RSPs vested (192,246 ) 1.26 Repurchased by Company (2,320 ) 2.27 Outstanding — December 31, 2018 47,051 $ 2.24 Market-Condition Awards Granted to Employees On August 11, 2017, our Board of Directors approved awards up to an aggregate of 365,250 RSUs to certain of our senior management team under the 2015 Plan, the vesting of which are contingent upon a combination of continued employment and achieving certain market capitalization milestones. The market-condition awards do not vest until the achievement of their respective market capitalization milestones, which must occur on or before December 31, 2019. The grant date fair value of these market-condition awards was estimated using a Monte Carlo simulation model. The derived service periods, which are the estimated periods of time that would be required to satisfy the market conditions, are also determined at the grant date. We record expense on a straight-line basis over the applicable derived service periods. The following table summarizes activity of the market-condition awards under the 2015 Plan and related information (in thousands, except share, per share amounts and vesting period): Number Weighted- Weighted- Aggregate Non-vested 353,250 $ 15.15 0.73 $ 13,900 Granted — — Vested (188,400 ) 18.22 Forfeited (5,600 ) 11.64 Non-vested 159,250 $ 11.64 0.04 $ 6,537 The following table summarizes the assumptions used to estimate the fair value of the market-condition awards during the year ended December 31, 2017: Valuation date stock price $ 28.55 Volatility 65.6 % Risk-free interest rate 1.4 % Dividend yield — At December 31, 2018, total unrecognized compensation expense related to unvested market-condition awards was $36,500, which is expected to be recognized over their respective remaining derived service periods. The remaining weighted average derived service period is 0.04 year as of December 31, 2018. We recognized $3.1 million and $2.2 million in stock-based compensation expense related to the market-condition awards for the year ended December 31, 2018 and December 31, 2017 respectively. Employee Stock Purchase Plan In July 2015, we adopted the 2015 Employee Stock Purchase Plan (the “2015 ESPP”). Under the 2015 ESPP our employees may purchase common stock through payroll deductions at a price equal to 85% of the lower of the fair market value of the stock at the beginning of the offering period or at the end of each applicable purchase period. The 2015 ESPP provided for offering periods of six months in duration. As approved by the Compensation Committee of the Board of Directors in December 2017, the 2015 ESPP provides for offering periods of two years in duration with purchase periods occurring every six months during an offering period. The purchase periods end on either January 31 or July 31. Contributions under the 2015 ESPP are limited to a maximum of 15% of an employee’s eligible compensation. ESPP purchases are settled with common stock from the ESPP’s previously authorized and available pool of shares. During the year ended December 31, 2018, 61,031 shares were issued under the ESPP for $1.6 million. The fair values of the rights granted under the 2015 ESPP were calculated using the following assumptions: Year Ended 8 Year Ended Expected term (in years) 0.5 – 2.0 0.5 Volatility 59.2-65.4 % 60.1-63.5 % Risk-free interest rate 1.6-2.7 % 0.7-1.2 % Dividend yield — % — % Stock-Based Compensation Expense Total stock-based compensation recognized by functions was as follows (in thousands): Year Ended December 31, 2018 2017 2016 Research and development $ 12,747 $ 5,905 $ 4,153 General and administrative 17,333 7,777 5,082 Total stock-based compensation expense $ 30,080 $ 13,682 $ 9,235 During the year ended December 31, 2016, we recorded charges of $1.5 million relating to the fair value of stock options which were modified for two terminated employees. $0.9 million of these charges were classified as research and development expenses and the remaining $0.6 million of these charges were classified as general and administrative expenses. Unrecognized Stock-Based Compensation Expense and Weighted-Average Remaining Amortization Period As of December 31, 2018 the unrecognized stock-based compensation cost, and the estimated weighted-average amortization period, using the straight-line attribution method, was as follows (in thousands, except amortization period): Unrecognized Compensation Cost Weighted-average remaining amortization period (years) Options $ 37,467 1.3 Restricted stock purchases 56 — Restricted stock units 31,466 1.1 ESPP 1,709 — Total unrecognized stock-based compensation expense $ 70,698 2.4

Defined Contribution Plan

Defined Contribution Plan	12 Months Ended
	Dec. 31, 2018
Postemployment Benefits [Abstract]
Defined Contribution Plan	8. Defined Contribution Plan In 2013 , we began to sponsor a 401 (k) retirement plan, in which substantially all of our full-time employees are eligible to participate. Eligible participants may contribute a percentage of their annual compensation to this plan, subject to statutory limitations. We made contributions to the Plan for eligible participants, and recorded contribution expenses of $0.8 million for the year ended December 31 , 2018 , $0.3 million for the year ended December 31 , 2017 and $0.2 million for the year ended December 31 , 2016 .

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	9. Income Taxes On December 22, 2017, the President signed the Tax Cuts and Jobs Act (the “Tax Act”). The Tax Act, among other things, lowered the U.S. corporate income tax rate from 35% to 21% effective January 1, 2018. Consequently, our gross deferred tax assets as of December 31, 2017 were significantly reduced to reflect the estimated impact of the Tax Act. Due to our lack of earnings history and uncertainties surrounding our ability to generate future taxable income, the net deferred tax assets have been fully offset by a valuation allowance. The significant reduction in our gross deferred tax assets are fully offset by a reduction in valuation allowance, resulting in no impact to our income tax expense. The components of the loss before income taxes were as follows (in thousands): Year Ended December 31, 2018 2017 2016 Loss before provision for income taxes: United States $ (174,190 ) $ (101,288 ) $ (70,103 ) International — (15,736 ) (12,365 ) $ (174,190 ) $ (117,024 ) $ (82,468 ) No provision for income taxes was recorded for the years incurred net operating losses for all the periods presented. We have not reflected any benefit of such net operating loss (NOL) carryforwards in the accompanying consolidated financial statements. We have established a full valuation allowance against the related deferred tax assets due to the uncertainty surrounding the realization of such assets. Our net operating loss carryforwards have been trued up to correctly reflect our NOL balance at the end of 2018. The true up is as a result of the Tax Act and subsequent changes to our U.S. international tax structure. The effective tax rate of the provision for income taxes differs from the federal statutory rate as follows: Year Ended December 31, 2018 2017 2016 Federal statutory income tax rate 21.0 % 34.0 % 34.0 % State taxes 1.3 — — Federal and state tax credits 7.1 7.3 8.3 Change in valuation allowance (33.3 ) (14.8 ) (41.1 ) Foreign rate differential 1.7 (4.6 ) (5.1 ) Officer compensation limitation (0.7 ) (0.9 ) — Stock based compensation/Non-deductible changes in fair value 2.9 0.7 3.9 Tax reform – tax rate change — (21.7 ) — Provision for Taxes 0.0 % 0.0 % 0.0 % The components of the deferred tax assets and liabilities are as follows (in thousands): December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 90,861 $ 51,448 Tax credits 47,375 32,732 Property and equipment 760 116 Accruals and reserves 1,954 1,668 Stock based compensation 4,838 1,915 Gross deferred tax assets 145,788 87,879 Valuation allowance (145,788 ) (87,879 ) Net deferred tax assets $ — $ — Realization of the deferred tax assets is dependent upon future taxable income, if any, the amount and timing of which are uncertain. We have established a valuation allowance to offset deferred tax assets as of December 31, 2018 and 2017 due to the uncertainty of realizing future tax benefits from our net operating loss and other deferred tax assets. The valuation allowance increased approximately $17.8 million, and $34.0 As of December 31, 2018, we had federal net operating loss carryforwards of approximately $381.9 million to offset future federal taxable income, with $208.9 million available through 2037 and $173.0 million available indefinitely. We also had state net operating loss carryforwards of approximately $151.7 million that may offset future state taxable income through 2036. Current federal and state tax laws include substantial restrictions on the utilization of net operating losses and tax credits in the event of an ownership change. Even if the carryforwards are available, they may be subject to annual limitations, lack of future taxable income, or future ownership changes that could result in the expiration of the carryforwards before they are utilized. At December 31, 2018, we recorded a 100% valuation allowance against our deferred tax assets of approximately $145.8 million, as at that time our management believed it was uncertain that they would be fully realized. If we determine in the future that we will be able to realize all or a portion of our net operating loss carryforwards, an adjustment to valuation allowance would increase net income in the period in which we make such a determination. In general, if we experience a greater than 50 percentage point aggregate change in ownership over a three-year period (a Section 382 ownership change), utilization of our pre-change NOL carryforwards are subject to an annual limitation under Section 382 of the Internal Revenue Code (California has similar laws). The annual limitation generally is determined by multiplying the value of our stock at the time of such ownership change (subject to certain adjustments) by the applicable long-term tax-exempt rate. Such limitations may result in expiration of a portion of the NOL carryforwards that were generated prior to 2018 before utilization. The NOL carryforwards that were generated during and after 2018 can be carried forward indefinitely and are able to offset up to 80% of taxable income in each future year. We have not utilized any NOL carryovers through December 31, 2018. In addition, our deferred tax assets are subject to full valuation allowance, and thus no benefit for deferred tax assets are recorded on our books. Our ability to use the remaining NOL carryforwards may be further limited if we experience a Section 382 ownership change as a result of future changes in our stock ownership. No liability related to uncertain tax positions is recorded on the consolidated financial statements. All uncertain tax positions are currently recorded as a reduction to our deferred tax asset. It is our policy to include penalties and interest expense related to income taxes as a component of other expense and interest expense, respectively, as necessary. A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands): December 31, 2018 2017 Balance at beginning of year $ 11,150 $ 5,296 Additions based on tax positions related to current year 5,144 6,346 Decreased for prior period positions (62 ) (492 ) Unrecognized tax benefit - December 31 $ 16,232 $ 11,150 We do not expect that our uncertain tax positions will materially change in the next twelve months. The reversal of the uncertain tax benefits will not impact our effective tax rate as we continue to maintain a full valuation allowance against our deferred tax assets. We file income tax returns in the United States, California and other states. We are not currently under examination by income tax authorities in federal, state or other jurisdictions. All tax returns will remain open for examination by the federal and state authorities for three and four years, respectively, from the date of utilization of any net operating loss or credits.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	10. Commitments and Contingencies Facilities In March 2017, we entered into a noncancelable operating lease (the “Lease”) for approximately 67,185 square feet of space in South San Francisco, California (the “Existing Premises”). The date on which we became responsible for paying rent under the Lease was December 15, 2017 (the “Rent Commencement Date”). The Lease expires 10 years after the Rent Commencement Date. The Lease grants us an option to extend the Lease for an additional 10-year 10-year In August 2018, we entered into an amendment to the Lease (the “Lease Amendment”) to relocate the leased premises from the Existing Premises to a to-be-constructed-building 10-year 10-year In March 2017, we provided a standby letter of credit of $0.9 million as security for our obligations under the Lease on our Existing Premises. The security deposit was increased to $2.4 million under the Lease Amendment. This standby letter of credit is classified as restricted cash. We intend to vacate the Existing Premises and surrender and deliver the Existing Premises to landlord on or before the date which is sixty days after the Substitute Premises Commencement Date, upon which time we will have no further obligations with respect to the Existing Premises. Upon signing of the Lease Amendment, we re-evaluated Future annual minimum lease payments due under the Lease and Lease Amendment at December 31 of each year are as follows (in thousands): Year ending December 31, Amount 1 2019 4,406 2020 6,513 2021 11,642 2022 12,020 Thereafter 102,776 Total $ 137,357 (1) The table above is prepared under the assumption that the Substitute Premises Commencement Date is June 30, 2020. Rent expense was $3.6 million for the year ended December 31, 2018, $2.0 million for the year ended December 31, 2017 and $1.3 million for the year ended December 31, 2016. The operating leases require us to share in prorated operating expenses and property taxes based upon actual amounts incurred; those amounts are not fixed for future periods and, therefore, are not included in the future commitments listed above. Indemnifications We indemnify each of our directors and officers for certain events or occurrences, subject to certain limits, while the director is or was serving at our request in such capacity, as permitted under Delaware law and in accordance with its certificate of incorporation and bylaws. The term of the indemnification period lasts as long as a director may be subject to any proceeding arising out of acts or omissions of such director in such capacity. The maximum amount of potential future indemnification is unlimited; however, we currently hold director liability insurance. This insurance allows the transfer of risk associated with our exposure and may enable us to recover a portion of any future amounts paid. We believe that the fair value of these indemnification obligations is minimal. Accordingly, we have not recognized any liabilities relating to these obligations for any period presented. Contingencies In the ordinary course of business, we may be subject to legal claims and regulatory actions that could have a material adverse effect on our business or financial position. We assess our potential liability in such situations by analyzing potential outcomes, assuming various litigation, regulatory and settlement strategies. If we determine a loss is probable and its amount can be reasonably estimated, we accrue an amount equal to the estimated loss. No losses and no provision for a loss contingency have been recorded to date. Contingent Payments In August 2018, we entered into a license agreement (the “License Agreement”) with F. Hoffmann-La Hoffmann-La know-how non-exclusive,

Net Loss per Share

Net Loss per Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Net Loss per Share	11. Net Loss per Share Basic net loss per share is computed by dividing net loss by the weighted-average number of common shares outstanding for the period. Since we were in a loss position for all periods presented, diluted net loss per share is the same as basic net loss per share for all periods as the inclusion of all potential common shares outstanding would have been anti-dilutive. The following securities were not included in the diluted net loss per share calculations because their effect was anti-dilutive: December 31, 2018 2017 2016 Options to purchase common stock 3,243,551 2,945,901 2,769,702 Restricted shares subject to future vesting 47,051 241,617 672,112 Restricted stock units 975,419 820,713 — Common stock potentially issuable for ESPP purchases — — 8,386 Total 4,266,021 4,008,231 3,450,200

Selected Quarterly Financial In

Selected Quarterly Financial Information (Unaudited)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Information (Unaudited)	Selected Quarterly Financial Information (unaudited) The following table provides the selected consolidated quarterly financial data for 2018 and 2017: Quarter Ended (in thousands, except per share December 31, 2018 September 30, June 30, March 31, December 31, September 30, June 30, March 31, Loss from operations $ (52,084 ) $ (45,476 ) $ (42,487 ) $ (42,695 ) $ (41,915 ) $ (29,180 ) $ (24,430 ) $ (23,721 ) Net loss $ (49,201 ) $ (43,068 ) $ (40,368 ) $ (41,556 ) $ (41,252 ) $ (28,557 ) $ (23,883 ) $ (23,332 ) Basic and diluted net loss per common share $ (0.93 ) $ (0.83 ) $ (0.78 ) $ (0.87 ) $ (0.95 ) $ (0.66 ) $ (0.55 ) $ (0.60 )

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”).
Use of Estimates	Use of Estimates The preparation of the accompanying consolidated financial statements in accordance with U.S. GAAP requires us to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the consolidated financial statements, and the reported amounts of costs and expenses during the reporting period. We base our estimates and assumptions on historical experience when available and on various factors that we believe to be reasonable under the circumstances. We evaluate our estimates and assumptions on an ongoing basis. Our actual results could differ from these estimates under different assumptions or conditions.
Principles of Consolidation	Principles of Consolidation The accompanying consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All significant intercompany transactions and balances have been eliminated upon consolidation.
Segment Reporting	Segment Reporting We have determined that we operate in a single segment based upon the way the business is organized for making operating decisions and assessing performance. The Company has only one operating segment related to the development of pharmaceutical products. All property and equipment is maintained in the United States.
Cash and Cash Equivalents	Cash and Cash Equivalents We consider all highly liquid investments with original maturities of three months or less at the time of purchase to be cash equivalents. Cash equivalents, which consist primarily of amounts invested in money market accounts, are stated at fair value.
Concentration of Credit Risk	Concentration of Credit Risk We invest in a variety of financial instruments and, by our policy, limit the amount of credit exposure with any one issuer, industry or geographic area for investments other than instruments backed by the U.S. federal government.
Investments in Marketable Securities	Investments in Marketable Securities We invest in marketable securities, primarily money market funds, corporate debt securities, government securities, government agency securities, and certificates of deposits. We classify our marketable securities as available-for-sale available-for-sale available-for-sale We regularly review all of our investments for other-than-temporary declines in estimated fair value. Our review includes the consideration of the cause of the impairment, including the creditworthiness of the security issuers, the number of securities in an unrealized loss position, the severity and duration of the unrealized losses, whether we have the intent to sell the securities and whether it is more likely than not that we will be required to sell the securities before the recovery of their amortized cost basis. When we determine that the decline in estimated fair value of an investment is below the amortized cost basis and the decline is other-than-temporary, we reduce the carrying value of the security and record a loss for the amount of such decline.
Fair Value Measurement	Fair Value Measurement The carrying amounts of certain financial instruments, including cash and cash equivalents, other receivables as included in other assets, current, restricted cash, accounts payable and accrued liabilities approximate fair value due to their relatively short maturities.
Property and Equipment, Net	Property and Equipment, Net Property and equipment are stated at cost, less accumulated depreciation and amortization. Depreciation is provided using the straight-line method over the estimated useful lives of the assets, three years for computer equipment and five years for laboratory equipment. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful life of the improvements. Depreciation and amortization begins at the time the asset is placed in service. Maintenance and repairs are charged as expense in the statements of operations and comprehensive loss as incurred. Upon sale or retirement of assets, the cost and related accumulated depreciation and amortization are removed from the consolidated balance sheet and any resulting gain or loss is reflected in operations.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets We evaluate our long-lived assets, including property and equipment, for impairment whenever events or changes in circumstances indicate that the carrying value of these assets may not be recoverable. Recoverability of these assets is measured by comparison of the carrying amount of each asset to the future undiscounted cash flows the asset is expected to generate over its remaining life. If the asset is considered to be impaired, the amount of any impairment is measured as the difference between the carrying value and the fair value of the impaired asset. There have been no impairments of our long-lived assets for the periods presented.
Restricted Cash	Restricted Cash Restricted cash consists of cash deposits held by our financial institution as collateral for our letter of credit under our facility lease.
Accruals of Research and Development Costs	Accruals of Research and Development Costs We record accruals for estimated costs of research, nonclinical and clinical studies and manufacturing development. These costs are a significant component of our research and development expenses. A substantial portion of our ongoing research and development activities are conducted by third-party service providers, including contract research organizations and contract manufacturing organizations. We accrue the costs incurred under our agreements with these third parties based on actual work completed in accordance with agreements established with these third parties. We determine the actual costs through discussions with internal personnel and external service providers as to the progress or stage of completion of the services and the agreed-upon fee to be paid for such services. We make judgments and estimates in determining the accrual balance in each reporting period. As actual costs become known, we adjust our accruals. We have not experienced any material deviations between accrued clinical trial expenses and actual clinical trial expenses. However, actual services performed, number of subjects enrolled, and the rate of subject enrollment may vary from our estimates, resulting in adjustments to clinical trial expense in future periods. Changes in these estimates that result in material changes to our accruals could materially affect our results of operations.
Leases	Leases We enter into lease agreements for our office and laboratory facilities. These leases are classified as operating leases. Rent expense is recognized on a straight-line basis over the noncancelable term of the lease and, accordingly, we record the difference between cash rent payments and the recognition of rent expense as a deferred rent liability, which is included within other liabilities on the consolidated balance sheet. Incentives granted under our facilities leases, including rent holiday and allowances to fund leasehold improvements, are deferred and are recognized as adjustments to rental expense on a straight-line basis over the noncancelable term of the lease.
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) is defined as a change in equity of a business enterprise during a period, resulting from transactions from non-owner
Research and Development	Research and Development Research and development costs are expensed as incurred and consist of salaries and benefits, stock-based compensation expense, lab supplies and facility costs, as well as fees paid to other nonemployees and entities that conduct certain research and development activities on our behalf. Amounts incurred in connection with license agreements are also included in research and development expense. Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are capitalized and then expensed as the related goods are delivered or the services are performed.
Stock-based Compensation	Stock-Based Compensation We measure and recognize stock-based compensation expense, including employee and non-employee
Income Taxes	Income Taxes We use the asset and liability method of accounting for income taxes. Under this method, deferred tax assets and liabilities are determined based on the differences between the financial reporting and the tax bases of assets and liabilities and are measured using the enacted tax rates and laws that will be in effect when the differences are expected to reverse. We must then assess the likelihood that the resulting deferred tax assets will be realized. A valuation allowance is provided when it is more likely than not that some portion or all of a deferred tax asset will not be realized. Due to our lack of earnings history, the net deferred tax assets have been fully offset by a valuation allowance. We recognize benefits of uncertain tax positions if it is more likely than not that such positions will be sustained upon examination based solely on their technical merits, as the largest amount of benefit that is more likely than not to be realized upon the ultimate settlement. It is our policy to include penalties and interest expense related to income taxes as a component of other expense and interest expense, respectively, as necessary. To date, there have been no interest or penalties incurred in relation to the unrecognized tax benefits.
Net Loss per Share	Net Loss per Share Basic net loss per share is calculated by dividing the net loss by the weighted average number of shares of common stock outstanding during the period, without consideration of common stock equivalents. Diluted net loss per share is the same as basic net loss per share, since the effects of potentially dilutive securities are antidilutive given our net loss.
Recent Accounting Pronouncements/Accounting Pronouncements Adopted	Recent Accounting Pronouncements Leases (Topic 842) In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842) right-of-use During 2018, the FASB issued ASU No. 2018-11, Leases (Topic 842) – Targeted Improvements The new standard also provides a number of optional practical expedients for the transition from ASC 840 – Leases (Topic 840) to Topic 842 that allow entities to not (i) reassess whether any expired or existing contracts are considered or contain leases; (ii) reassess the lease classification for any expired or existing leases; and (iii) reassess initial direct costs for any existing leases. These practical expedients, if elected, must be elected as a package and applied consistently by an entity to all of its leases. We plan to elect the use of practical expedients as a package. We expect that this standard will have a material effect on our consolidated financial statements. While we continue to evaluate the provisions of ASC 842 to determine the impact the adoption will have on our consolidated financial statements, we currently believe the most significant effects relate to the recognition of new right-of-use (ROU) assets and lease liabilities on our consolidated balance sheet related to our office and equipment operating leases. We do not expect a significant change in our leasing activities between now and adoption. On adoption, we currently expect to recognize additional lease liabilities of approximately $25.9 million and corresponding ROU assets of approximately $14.2 million, which reflect lease incentives previously received of approximately $11.7 The new standard also provides practical expedients for an entity’s ongoing accounting. We currently expect to elect the short-term lease recognition exemption for all leases that qualify. This means, for those leases that qualify, we will not recognize ROU assets or lease liabilities, and this includes not recognizing ROU assets or lease liabilities for existing short-term leases of those assets in transition. We also currently expect to elect the practical expedient to not separate lease and non-lease components for all of our leases. Other recent accounting pronouncements In February 2018, the FASB issued ASU No. 2018-02, Income Statement – Reporting Comprehensive Income (Topic 220): Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income In August 2018, the FASB issued ASU No. 2018-13, Fair Value Measurement (Topic 820) In August 2018, the FASB issued ASU No. 2018-15, Intangibles – Goodwill and Other – Internal-Use Software (Subtopic 350-40), Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract Accounting Pronouncements Adopted In August 2016, the FASB issued ASU No. 2016-15, Classification of Certain Cash Receipts and Cash Payments In November 2016, the FASB issued ASU No. 2016-18, Restricted Cash In May 2017, the FASB issued ASU No. 2017-09, Compensation – Stock Compensation (Topic 718) In March 2018, the FASB issued ASU No. 2018-04, Investments – Debt Securities (Topic 320) and Regulated Operations (Topic 980): Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 117 and SEC Release No. 33-9273. Investments-Debt Securities Regulated Operations In March 2018, the FASB issued ASU No. 2018-05, Income Taxes (Topic 740): Amendments to SEC Paragraphs Pursuant to SEC Staff Accounting Bulletin No. 118 Income Taxes In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718), Improvements to Nonemployee Share-Based Payment Accounting

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value, Assets Measured on Recurring Basis	The following table summarizes our financial assets measured at fair value on a recurring basis (in thousands): December 31, 2018 Total Level 1 Level 2 Level 3 Financial Assets: Money market funds $ 275,234 $ 275,234 $ — $ — Corporate debt securities 110,027 — 110,027 — U.S. government agency securities 88,028 — 88,028 — Certificates of deposits 6,675 — 6,675 — U.S. government securities 111,728 — 111,728 — Total financial assets $ 591,692 $ 275,234 $ 316,458 $ — December 31, 2017 Total Level 1 Level 2 Level 3 Financial Assets: Money market funds $ 185,824 $ 185,824 1 $ — $ — Corporate debt securities 46,977 — 46,977 — U.S. government agency securities 54,989 — 54,989 — Certificates of deposits 9,129 — 9,129 — U.S. government securities 20,007 — 20,007 — Total financial assets $ 316,926 $ 185,824 $ 131,102 $ —

Available-for-Sale Securities (

Available-for-Sale Securities (Tables)	12 Months Ended
	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Summary of Available-for-Sale Securities	The following table is a summary of available-for-sale December 31, 2018 December 31, 2017 Amortized Cost Unrealized Gains Unrealized (Losses) Estimated Fair Value Amortized Cost Unrealized Gains Unrealized (Losses) Estimated Fair Value Financial Assets: Money market funds $ 275,234 $ — $ — $ 275,234 $ 185,824 $ — $ — $ 185,824 1 Corporate debt securities 110,053 69 (95 ) 110,027 47,108 — (131 ) 46,977 U.S. government agency securities 88,042 40 (54 ) 88,028 55,170 — (181 ) 54,989 Certificates of deposits 6,681 1 (7 ) 6,675 9,142 — (13 ) 9,129 U.S. government securities 111,730 60 (62 ) 111,728 20,018 — (11 ) 20,007 Total $ 591,740 $ 170 $ (218 ) $ 591,692 $ 317,262 $ — $ (336 ) $ 316,926 (1) In 2017, some balances have been reclassified from cash to money market funds in the above table.
Summary of Classification of Available-for-Sale Securities on Consolidated Balance Sheets	The following table summarizes the classification of the available-for-sale securities on our Consolidated Balance Sheets (in thousands): December 31, 2018 December 31, 2017 Cash and cash equivalents $ 275,234 $ 185,826 1 Short-term marketable securities 202,177 116,493 Long-term marketable securities 114,281 14,607 Total $ 591,692 $ 316,926 (1) In 2017, some balances have been reclassified from cash to money market funds in the above table.

Balance Sheet Components (Table

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Property and Equipment	Property and equipment consist of the following (in thousands): December 31, 2018 2017 Laboratory equipment $ 7,363 $ 5,715 Computer equipment 1,501 1,594 Leasehold improvements 13,785 12,642 Construction-in-progress 239 419 Total property and equipment 22,888 20,370 Less: accumulated depreciation and amortization (7,907 ) (3,799 ) Property and equipment, net $ 14,981 $ 16,571
Accrued Liabilities	Accrued liabilities consist of the following (in thousands): December 31, 2018 2017 Accrued clinical and manufacturing expenses $ 15,121 $ 8,035 Accrued professional and consulting services 1,016 1,007 Other 655 1,093 Total accrued liabilities $ 16,792 $ 10,135
Other Liabilities	Other liabilities consist of the following (in thousands): December 31, 2018 2017 Restricted shares subject to repurchase, current $ 157 $ 373 Deferred rent, current 712 — Other payable, current 30 — Total other liabilities, current $ 899 $ 373 Restricted shares subject to repurchase, noncurrent $ — $ 161 Deferred rent, noncurrent 11,041 11,491 Other payable, noncurrent 30 — Total other liabilities, noncurrent $ 11,071 $ 11,652

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Schedule of Common Stock Reserved For Future Issuance	We have reserved shares of common stock, on an as-converted December 31, 2018 2017 Restricted shares subject to future vesting 47,051 241,617 Restricted stock units 975,419 820,713 Options issued and outstanding 3,243,551 2,945,901 Shares available for future grant under the 2015 Plan and 2017 Inducement Equity Plan 3,003,454 1,708,680 Employee stock purchase plan 240,935 186,033 Total 7,510,410 5,902,944
Schedule of Restricted Shares Subject to Repurchase	Restricted shares subject to repurchase were as follows: December 31, 2018 2017 Restricted shares subject to repurchase: Shares issued pursuant to the 2012 Stock Option and Grant Plan 47,051 241,617 Total restricted shares subject to repurchase 47,051 241,617

Share-based Compensation (Table

Share-based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Stock Option Activity	The following table summarizes activity under the Company’s stock option plans, including the 2017 Inducement Plan, 2015 Plan and the 2012 Plan and related information (in thousands, except share and per share amounts and term): Number Weighted- Weighted- Aggregate Outstanding — December 31, 2017 2,945,901 $ 17.50 8.25 Options granted 1,102,208 52.80 Options exercised (596,434 ) 10.10 Options canceled (208,124 ) 34.84 Outstanding — December 31, 2018 3,243,551 $ 29.74 7.96 $ 49,373 Vested and exercisable — December 31, 2018 1,553,778 $ 21.80 7.31 $ 32,305
Valuation Assumptions for Stock Awards	The fair values of stock options granted to employees were calculated using the following assumptions: Year Ended December 31, 2018 2017 2016 Expected term (in years) 5.3-6.1 5.3-6.1 5.3-6.1 Volatility 68.7%-71.8% 68.9%-75.6% 70.6%-82.3% Risk-free interest rate 2.6%-3.0% 1.8%-2.3% 1.1%-2.1% Dividend yield — — —
Stock-based Compensation Expense Recognized	Total stock-based compensation recognized by functions was as follows (in thousands): Year Ended December 31, 2018 2017 2016 Research and development $ 12,747 $ 5,905 $ 4,153 General and administrative 17,333 7,777 5,082 Total stock-based compensation expense $ 30,080 $ 13,682 $ 9,235
Unrecognized Stock-based Compensation Cost	As of December 31, 2018 the unrecognized stock-based compensation cost, and the estimated weighted-average amortization period, using the straight-line attribution method, was as follows (in thousands, except amortization period): Unrecognized Compensation Cost Weighted-average remaining amortization period (years) Options $ 37,467 1.3 Restricted stock purchases 56 — Restricted stock units 31,466 1.1 ESPP 1,709 — Total unrecognized stock-based compensation expense $ 70,698 2.4
Restricted Stock Units [Member]
Non-Vested Restricted Stock Activity	The following table summarizes activity of RSUs granted to employees with service-based vesting under the 2017 Inducement Plan and 2015 Plan and related information (in thousands, except share, per share amounts and vesting period): Number Weighted- Weighted- Aggregate Non-vested 467,463 $ 24.93 1.71 $ 18,395 RSUs granted 625,765 54.48 RSUs vested (179,772 ) 34.38 RSUs forfeited (97,287 ) 41.89 Non-vested 816,169 $ 43.34 1.54 $ 33,504
Market Condition Awards Granted to Employees [Member]
Summary of Activity of Market-Condition Awards	The following table summarizes activity of the market-condition awards under the 2015 Plan and related information (in thousands, except share, per share amounts and vesting period): Number Weighted- Weighted- Aggregate Non-vested 353,250 $ 15.15 0.73 $ 13,900 Granted — — Vested (188,400 ) 18.22 Forfeited (5,600 ) 11.64 Non-vested 159,250 $ 11.64 0.04 $ 6,537
Valuation Assumptions of Market-Condition Awards	The following table summarizes the assumptions used to estimate the fair value of the market-condition awards during the year ended December 31, 2017: Valuation date stock price $ 28.55 Volatility 65.6 % Risk-free interest rate 1.4 % Dividend yield —
Restricted Stock Purchases [Member]
Non-Vested Restricted Stock Activity	A summary of our unvested restricted stock for the year ended December 31, 2018 is as follows: Number of RSPs Weighted Average Outstanding — December 31, 2017 241,617 $ 1.46 RSPs vested (192,246 ) 1.26 Repurchased by Company (2,320 ) 2.27 Outstanding — December 31, 2018 47,051 $ 2.24
ESPP [Member]
Fair Value Assumptions for Employee Stock Purchase Plan	The fair values of the rights granted under the 2015 ESPP were calculated using the following assumptions: Year Ended 8 Year Ended Expected term (in years) 0.5 – 2.0 0.5 Volatility 59.2-65.4 % 60.1-63.5 % Risk-free interest rate 1.6-2.7 % 0.7-1.2 % Dividend yield — % — %

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Components of Loss Before Income Taxes	The components of the loss before income taxes were as follows (in thousands): Year Ended December 31, 2018 2017 2016 Loss before provision for income taxes: United States $ (174,190 ) $ (101,288 ) $ (70,103 ) International — (15,736 ) (12,365 ) $ (174,190 ) $ (117,024 ) $ (82,468 )
Effective Income Tax Rate Reconciliation	The effective tax rate of the provision for income taxes differs from the federal statutory rate as follows: Year Ended December 31, 2018 2017 2016 Federal statutory income tax rate 21.0 % 34.0 % 34.0 % State taxes 1.3 — — Federal and state tax credits 7.1 7.3 8.3 Change in valuation allowance (33.3 ) (14.8 ) (41.1 ) Foreign rate differential 1.7 (4.6 ) (5.1 ) Officer compensation limitation (0.7 ) (0.9 ) — Stock based compensation/Non-deductible changes in fair value 2.9 0.7 3.9 Tax reform – tax rate change — (21.7 ) — Provision for Taxes 0.0 % 0.0 % 0.0 %
Components of Deferred Tax Assets and Liabilities	The components of the deferred tax assets and liabilities are as follows (in thousands): December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 90,861 $ 51,448 Tax credits 47,375 32,732 Property and equipment 760 116 Accruals and reserves 1,954 1,668 Stock based compensation 4,838 1,915 Gross deferred tax assets 145,788 87,879 Valuation allowance (145,788 ) (87,879 ) Net deferred tax assets $ — $ —
Reconciliation of Unrecognized Tax Benefits	A reconciliation of the beginning and ending amount of unrecognized tax benefits is as follows (in thousands): December 31, 2018 2017 Balance at beginning of year $ 11,150 $ 5,296 Additions based on tax positions related to current year 5,144 6,346 Decreased for prior period positions (62 ) (492 ) Unrecognized tax benefit - December 31 $ 16,232 $ 11,150

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments and Contingencies Disclosure [Abstract]
Future Aggregate Minimum Lease Payments	Future annual minimum lease payments due under the Lease and Lease Amendment at December 31 of each year are as follows (in thousands): Year ending December 31, Amount 1 2019 4,406 2020 6,513 2021 11,642 2022 12,020 Thereafter 102,776 Total $ 137,357 (1) The table above is prepared under the assumption that the Substitute Premises Commencement Date is June 30, 2020.

Net Loss per Share (Tables)

Net Loss per Share (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Potentially Dilutive Securities that were Not Included in Diluted Net Loss per Share Calculations	The following securities were not included in the diluted net loss per share calculations because their effect was anti-dilutive: December 31, 2018 2017 2016 Options to purchase common stock 3,243,551 2,945,901 2,769,702 Restricted shares subject to future vesting 47,051 241,617 672,112 Restricted stock units 975,419 820,713 — Common stock potentially issuable for ESPP purchases — — 8,386 Total 4,266,021 4,008,231 3,450,200

Selected Quarterly Financial _2

Selected Quarterly Financial Information (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Selected Consolidated Quarterly Financial Information (Unaudited)	The following table provides the selected consolidated quarterly financial data for 2018 and 2017: Quarter Ended (in thousands, except per share December 31, 2018 September 30, June 30, March 31, December 31, September 30, June 30, March 31, Loss from operations $ (52,084 ) $ (45,476 ) $ (42,487 ) $ (42,695 ) $ (41,915 ) $ (29,180 ) $ (24,430 ) $ (23,721 ) Net loss $ (49,201 ) $ (43,068 ) $ (40,368 ) $ (41,556 ) $ (41,252 ) $ (28,557 ) $ (23,883 ) $ (23,332 ) Basic and diluted net loss per common share $ (0.93 ) $ (0.83 ) $ (0.78 ) $ (0.87 ) $ (0.95 ) $ (0.66 ) $ (0.55 ) $ (0.60 )

Organization and Basis of Pre_2

Organization and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands	1 Months Ended					12 Months Ended
	Jan. 31, 2019USD ($)$ / sharesshares	Dec. 31, 2018USD ($)$ / sharesshares	Mar. 31, 2018USD ($)$ / sharesshares	Jan. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2018USD ($)Segment$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)shares
Class of Stock [Line Items]
Number of operating segments | Segment						1
Proceeds from issuance of common stock, net of issuance costs						$ 396,501	$ 231,955	$ 116,995
Cumulative net losses		$ (472,150)			$ (297,957)	$ (472,150)	$ (297,957)
Common Stock [Member]
Class of Stock [Line Items]
Common stock shares sold | shares		3,409,090	4,600,000		2,631,579	8,403,826	8,498,926	6,667,228
Share price per share | $ / shares		$ 44	$ 54		$ 38	$ 44	$ 38
Proceeds from issuance of common stock, net of issuance costs		$ 140,900			$ 96,400
Common Stock [Member] | Over-Allotment Option [Member]
Class of Stock [Line Items]
Common stock shares sold | shares			600,000	394,736
Share price per share | $ / shares			$ 54	$ 38
Proceeds from issuance of common stock, net of issuance costs			$ 240,600	$ 14,600
Common Stock [Member] | Over-Allotment Option [Member] | Subsequent Event [Member]
Class of Stock [Line Items]
Common stock shares sold | shares	511,363
Share price per share | $ / shares	$ 44
Proceeds from issuance of common stock, net of issuance costs	$ 21,200

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Summary Of Significant Accounting Policies [Line Items]
Impairment of long-lived assets	$ 0	$ 0	$ 0
Operating Lease Liability	25,900,000
Deferred Rent Liability	11,700,000
Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Operating Lease Liability	25,900,000
Operating Lease Right Of Use Asset	$ 14,200,000
Computer Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	3 years
Laboratory Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	5 years

Fair Value Measurements - Fair

Fair Value Measurements - Fair Value Assets Measured on Recurring Basis (Detail) - Fair Value Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	$ 591,692	$ 316,926
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	111,728	20,007
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	275,234	185,824
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	316,458	131,102
Level 2 [Member] | U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	111,728	20,007
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	275,234	185,824
Money Market Funds [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	275,234	185,824	[1]
Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	110,027	46,977
Corporate Debt Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	110,027	46,977
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	88,028	54,989
U.S. Government Agency Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	88,028	54,989
Certificates of Deposits [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	6,675	9,129
Certificates of Deposits [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	$ 6,675	$ 9,129
[1]	In 2017, some balances have been reclassified from cash to money market funds in the above table.

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Detail)	Dec. 31, 2018USD ($)
Fair Value Disclosures [Abstract]
Fair value assets amount transfer from level 1 to level 2	$ 0
Fair value assets amount transfer from level 2 to level 1	$ 0

Available-for-Sale Securities -

Available-for-Sale Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Available for sale securities, Amortized Cost	$ 591,740	$ 317,262
Available for sale securities, Unrealized Gains	170
Available for sale securities, Unrealized (Losses)	(218)	(336)
Available for sale securities, Estimated Fair Value	591,692	316,926
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available for sale securities, Amortized Cost	110,053	47,108
Available for sale securities, Unrealized Gains	69
Available for sale securities, Unrealized (Losses)	(95)	(131)
Available for sale securities, Estimated Fair Value	110,027	46,977
U.S. Government Agency Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available for sale securities, Amortized Cost	88,042	55,170
Available for sale securities, Unrealized Gains	40
Available for sale securities, Unrealized (Losses)	(54)	(181)
Available for sale securities, Estimated Fair Value	88,028	54,989
Money Market Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Available for sale securities, Amortized Cost	275,234	185,824
Available for sale securities, Estimated Fair Value	275,234	185,824	[1]
Certificates of Deposits [Member]
Debt Securities, Available-for-sale [Line Items]
Available for sale securities, Amortized Cost	6,681	9,142
Available for sale securities, Unrealized Gains	1
Available for sale securities, Unrealized (Losses)	(7)	(13)
Available for sale securities, Estimated Fair Value	6,675	9,129
U.S. Government Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available for sale securities, Amortized Cost	111,730	20,018
Available for sale securities, Unrealized Gains	60
Available for sale securities, Unrealized (Losses)	(62)	(11)
Available for sale securities, Estimated Fair Value	$ 111,728	$ 20,007
[1]	In 2017, some balances have been reclassified from cash to money market funds in the above table.

Available-for-Sale Securities_2

Available-for-Sale Securities - Summary of Classification of Available-for-Sale Securities on Consolidated Balance Sheets (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Cash and cash equivalents	$ 275,234	$ 185,826	[1]
Short-term marketable securities	202,177	116,493
Long-term marketable securities	114,281	14,607
Total	$ 591,692	$ 316,926
[1]	In 2017, some balances have been reclassified from cash to money market funds in the above table.

Balance Sheet Components - Prop

Balance Sheet Components - Property and Equipment (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 22,888	$ 20,370
Less: accumulated depreciation and amortization	(7,907)	(3,799)
Property and equipment, net	14,981	16,571
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	7,363	5,715
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	1,501	1,594
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	13,785	12,642
Construction-in-Progress [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 239	$ 419

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Balance Sheet Components [Abstract]
Depreciation expense	$ 4.7	$ 1.7	$ 1.2

Balance Sheet Components - Accr

Balance Sheet Components - Accrued Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Balance Sheet Components [Abstract]
Accrued clinical and manufacturing expenses	$ 15,121	$ 8,035
Accrued professional and consulting services	1,016	1,007
Other	655	1,093
Total accrued liabilities	$ 16,792	$ 10,135

Balance Sheet Components - Othe

Balance Sheet Components - Other Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Balance Sheet Components [Abstract]
Restricted shares subject to repurchase,current	$ 157	$ 373
Deferred rent, current	712
Other payable, current	30
Total other liabilities, current	899	373
Restricted shares subject to repurchase, noncurrent		161
Deferred rent, noncurrent	11,041	11,491
Other payable, noncurrent	30
Total other liabilities, noncurrent	$ 11,071	$ 11,652

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017USD ($)
Equity [Abstract]
Common stock dividend declared	$ 0

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule of Common Stock Reserved for Issuance on Converted basis (Detail) - shares	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Shares reserved for future issuance	7,510,410	5,902,944
Restricted Stock [Member]
Class of Stock [Line Items]
Shares reserved for future issuance	47,051	241,617
RSU [Member]
Class of Stock [Line Items]
Shares reserved for future issuance	975,419	820,713
Stock Options [Member]
Class of Stock [Line Items]
Shares reserved for future issuance	3,243,551	2,945,901
Shares Available for Future Grant Under the 2015 Plan and 2017 Inducement Equity Plan [Member]
Class of Stock [Line Items]
Shares reserved for future issuance	3,003,454	1,708,680
ESPP [Member]
Class of Stock [Line Items]
Shares reserved for future issuance	240,935	186,033

Stockholders' Equity - Schedu_2

Stockholders' Equity - Schedule of Restricted Shares Subject to Repurchase (Detail) - shares	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Total restricted shares subject to repurchase	47,051	241,617
Restricted Stock [Member]
Class of Stock [Line Items]
Shares issued pursuant to the 2012 Stock Option and Grant Plan	47,051	241,617
2012 Stock Option and Grant Plan [Member] | Restricted Stock [Member]
Class of Stock [Line Items]
Shares issued pursuant to the 2012 Stock Option and Grant Plan	47,051	241,617

Share-based Compensation - Plan

Share-based Compensation - Plan Summary - Additional Information (Detail)	Aug. 11, 2017shares	Apr. 09, 2015shares	Jul. 31, 2015	Dec. 31, 2018USD ($)$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)$ / shares	Dec. 31, 2012shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance | shares				7,510,410	5,902,944
Intrinsic value of options exercised				$ 23,400,000	$ 11,300,000	$ 2,400,000
Estimated weighted-average grant-date fair value of common stock underlying options granted | $ / shares				$ 33.58	$ 16.19	$ 11.74
Dividend yield				0.00%	0.00%
Unrecognized compensation expense				$ 70,698,000
weighted average derived service period				2 years 4 months 24 days
Stock-based compensation				$ 30,080,000	$ 13,682,000	$ 9,235,000
Shares issued, value				1,647,000	1,050,000	1,018,000
Total stock-based compensation expense				30,080,000	13,682,000	$ 9,235,000
Number of employees affected						2
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense				12,747,000	5,905,000	$ 4,153,000
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense				$ 17,333,000	$ 7,777,000	$ 5,082,000
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance | shares				3,243,551	2,945,901
Dividend yield				0.00%	0.00%	0.00%
weighted average derived service period				1 year 3 months 18 days
ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance | shares				240,935	186,033
weighted average derived service period				0 years
RSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance | shares				975,419	820,713
weighted average derived service period				1 year 1 month 6 days
Market Condition Awards Granted to Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield				0.00%
Unrecognized compensation expense				$ 36,500
weighted average derived service period				14 days
Stock-based compensation				$ 3,100,000	$ 2,200,000
Market Condition Awards Granted to Employees [Member] | Chief Executive Officer [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Minimum consecutive trading days for market capitalization		20 days
2017 Inducement Equity Plan [Member] | Stock Compensation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance | shares				458,350
2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance | shares				2,545,104
2015 Plan [Member] | Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration period				10 years
Exercise price as a percentage of the fair market value				100.00%
Exercise price as a percentage of the fair market value for option holding more than 10% total combined voting power				110.00%
Vesting period				4 years
2015 Plan [Member] | RSU [Member] | Senior Management [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares granted in period | shares	365,250
2015 Plan [Member] | Market Condition Awards Granted to Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares granted in period | shares				0
2012 Stock Option and Grant Plan [Member] | ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance | shares		3,785,713					2,785,713
Expiration period				10 years
Increase in shares authorized for issuance under plan (shares) | shares		1,000,000
2015 ESPP [Member] | ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Exercise price as a percentage of the fair market value			85.00%
Offering period			6 years
Maximum contribution to plan as a percent of employee's eligible compensation			15.00%
Shares issued | shares				61,031
Shares issued, value				$ 1,600,000
Stock Option Modified [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense						$ 1,500,000
Stock Option Modified [Member] | Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense						900,000
Stock Option Modified [Member] | General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total stock-based compensation expense						$ 600,000

Share-based Compensation - Stoc

Share-based Compensation - Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning balance, outstanding, Number of Options	2,945,901
Options granted, Number of Options	1,102,208
Options exercised, Number of Options	(596,434)
Options canceled, Number of Options	(208,124)
Ending balance, outstanding, Number of Options	3,243,551	2,945,901
Vested and exercisable, Number of Options	1,553,778
Weighted- Average Exercise Price
Beginning balance, Outstanding, Weighted-Average Exercise Price	$ 17.50
Options granted, Weighted-Average Exercise Price	52.80
Options exercised, Weighted-Average Exercise Price	10.10
Options canceled, Weighted-Average Exercise Price	34.84
Ending balance, Outstanding, Weighted-Average Exercise Price	29.74	$ 17.50
Ending balance, Outstanding, Weighted-Average Exercise Price	$ 21.80
Beginning, Outstanding, Weighted-Average remaining contractual term	7 years 11 months 16 days	8 years 3 months
Vested and exercisable, Weighted-Average remaining contractual term	7 years 3 months 22 days
Balance Outstanding, Aggregate Intrinsic Value	$ 49,373
Vested and exercisable, Aggregate Intrinsic Value	$ 32,305

Share-based Compensation - Valu

Share-based Compensation - Valuation Assumptions for Stock Options Granted (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.00%	0.00%
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility, minimum	68.70%	68.90%	70.60%
Volatility, maximum	71.80%	75.60%	82.30%
Risk-free interest rate, minimum	2.60%	1.80%	1.10%
Risk-free interest rate, maximum	3.00%	2.30%	2.10%
Dividend yield	0.00%	0.00%	0.00%
Stock Options [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	5 years 3 months 18 days	5 years 3 months 18 days	5 years 3 months 18 days
Stock Options [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days

Share-based Compensation - Summ

Share-based Compensation - Summary of RSU Activity Granted to Employees with Service-Based Vesting (Detail) - 2017 Inducement Plan and 2015 Plan [Member] - RSU [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance - Non-vested market-condition awards	467,463
RSUs granted, Number of RSUs	625,765
RSUs vested, Number of RSUs	(179,772)
RSUs forfeited, Number of RSUs	(97,287)
Ending Balance - Non-vested market-condition awards	816,169	467,463
Beginning Balance - Non-vested units, Weighted-Average Grant Date Fair Value	$ 24.93
RSUs granted, Weighted-Average Grant Date Fair Value	54.48
RSUs vested, Weighted-Average Grant Date Fair Value	34.38
RSUs forfeited, Weighted-Average Grant Date Fair Value	41.89
Ending Balance - Non-vested units, Weighted-Average Grant Date Fair Value	$ 43.34	$ 24.93
Non-vested units, Weighted-Average remaining vesting period(years)	1 year 6 months 14 days	1 year 8 months 16 days
Non-vested units, Aggregate Intrinsic Value	$ 33,504	$ 18,395

Share-based Compensation - Unve

Share-based Compensation - Unvested Restricted Stock Activity (Detail) - Restricted Stock [Member]	12 Months Ended
	Dec. 31, 2018$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance - Non-vested market-condition awards | shares	241,617
Number of units vested | shares	(192,246)
Number of RSPs Repurchased by Company | shares	(2,320)
Ending Balance - Non-vested market-condition awards | shares	47,051
Beginning Balance - Non-vested units, Weighted-Average Grant Date Fair Value | $ / shares	$ 1.46
Weighted Average Grant Date Fair Value per Share, RSPs vested | $ / shares	1.26
Weighted Average Grant Date Fair Value per Share, Repurchased by Company | $ / shares	2.27
Ending Balance - Non-vested units, Weighted-Average Grant Date Fair Value | $ / shares	$ 2.24

Share-based Compensation - Su_2

Share-based Compensation - Summary of Activity of Market-Condition Awards (Detail) - Market Condition Awards Granted to Employees [Member] - 2015 Plan [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance - Non-vested market-condition awards	353,250
Number of units granted	0
Number of units vested	(188,400)
Number of units forfeited	(5,600)
Ending Balance - Non-vested market-condition awards	159,250	353,250
Beginning Balance - Non-vested units, Weighted-Average Grant Date Fair Value	$ 15.15
Weighted-Average Grant Date Fair Value granted	0
Weighted-Average Grant Date Fair Value vested	18.22
Weighted-Average Grant Date Fair Value forfeited	11.64
Ending Balance - Non-vested units, Weighted-Average Grant Date Fair Value	$ 11.64	$ 15.15
Non-vested units, Weighted-Average remaining vesting period(years)	14 days	8 months 23 days
Non-vested units, Aggregate Intrinsic Value	$ 6,537	$ 13,900

Share-based Compensation - Va_2

Share-based Compensation - Valuation Assumptions of Market-Condition Awards (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.00%	0.00%
Market Condition Awards Granted to Employees [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Valuation date stock price		$ 28.55
Volatility		65.60%
Risk-free interest rate		1.40%
Dividend yield	0.00%

Share-based Compensation - Fair

Share-based Compensation - Fair Value Assumptions for Employee Stock Purchase Plan (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Dividend yield	0.00%	0.00%
ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)		6 months
Volatility	0.00%	0.00%
Volatility, minimum	59.20%	60.10%
Volatility, maximum	65.40%	63.50%
Risk-free interest rate	0.00%	0.00%
Risk-free interest rate, minimum	1.60%	0.70%
Risk-free interest rate, maximum	2.70%	1.20%
ESPP [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	6 months
ESPP [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term (in years)	2 years

Share-based Compensation - St_2

Share-based Compensation - Stock-based Compensation Expense Recognized (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 30,080	$ 13,682	$ 9,235
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	12,747	5,905	4,153
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 17,333	$ 7,777	$ 5,082

Share-based Compensation - Unre

Share-based Compensation - Unrecognized Stock-based Compensation Cost (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2018USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total unrecognized stock-based compensation expense	$ 70,698
Weighted-average remaining amortization period	2 years 4 months 24 days
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost, options	$ 37,467
Weighted-average remaining amortization period	1 year 3 months 18 days
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost, restricted stock awards	$ 56
Weighted-average remaining amortization period	0 years
RSU [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost, restricted stock awards	$ 31,466
Weighted-average remaining amortization period	1 year 1 month 6 days
ESPP [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized compensation cost, restricted stock awards	$ 1,709
Weighted-average remaining amortization period	0 years

Defined Contribution Plan - Add

Defined Contribution Plan - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Retirement Benefits [Abstract]
Contribution expenses	$ 0.8	$ 0.3	$ 0.2

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
U.S. corporate income tax rate	21.00%	34.00%	34.00%
Provision for income taxes		$ 0	$ 0	$ 0
Increase in valuation allowance	$ 57,900,000	17,800,000	$ 34,000,000
Deferred tax assets, valuation allowance	$ 145,788,000	$ 87,879,000
valuation allowance charged on deferred tax assets	100.00%
Portion of taxable income to be offset by NOL carry forwards	80.00%
Tax Cuts and Jobs Act [Member]
Income Taxes [Line Items]
U.S. corporate income tax rate	21.00%	35.00%
State [Member]
Income Taxes [Line Items]
Operating loss carry forward, expiration year	2,036
State [Member] | Operating Loss Carryforward Expiration Year 2036 [Member]
Income Taxes [Line Items]
Operating loss carryforwards	$ 151,700,000
Domestic Tax Authority [Member]
Income Taxes [Line Items]
Operating loss carryforwards	$ 381,900,000
Operating loss carry forward, expiration year	2,037
Domestic Tax Authority [Member] | Tax period 2037 [Member]
Income Taxes [Line Items]
Operating loss carryforwards	$ 208,900,000
Domestic Tax Authority [Member] | Operating Loss Carryforward With Indefinite Expiry Period [Member]
Income Taxes [Line Items]
Operating loss carryforwards	$ 173,000,000

Income Taxes - Components of Lo

Income Taxes - Components of Loss Before Income Taxes (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Loss before provision for income taxes:
United States	$ (174,190)	$ (101,288)	$ (70,103)
International		(15,736)	(12,365)
Loss before provision for income taxes	$ (174,190)	$ (117,024)	$ (82,468)

Income Taxes - Effective Income

Income Taxes - Effective Income Tax Rate Reconciliation (Detail)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Federal statutory income tax rate	21.00%	34.00%	34.00%
State taxes	1.30%
Federal and state tax credits	7.10%	7.30%	8.30%
Change in valuation allowance	(33.30%)	(14.80%)	(41.10%)
Foreign rate differential	1.70%	(4.60%)	(5.10%)
Officer compensation limitation	(0.70%)	(0.90%)
Stock based compensation/Non-deductible changes in fair value	2.90%	0.70%	3.90%
Tax reform - tax rate change		(21.70%)
Provision for Taxes	0.00%	0.00%	0.00%

Income Taxes - Components of De

Income Taxes - Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 90,861	$ 51,448
Tax credits	47,375	32,732
Property and equipment	760	116
Accruals and reserves	1,954	1,668
Stock based compensation	4,838	1,915
Gross deferred tax assets	145,788	87,879
Valuation allowance	(145,788)	(87,879)
Net deferred tax assets	$ 0	$ 0

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ 11,150	$ 5,296
Additions based on tax positions related to current year	5,144	6,346
Decreased for prior period positions	(62)	(492)
Unrecognized tax benefit - December 31	$ 16,232	$ 11,150

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail)	1 Months Ended		12 Months Ended
	Aug. 31, 2018USD ($)ft²	Mar. 31, 2017USD ($)ft²	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Commitment And Contingencies [Line Items]
Facility size | ft²	164,150	67,185
Initial term	10 years	10 years
Future minimum rental payments under the Lease term	$ 121,500,000	$ 48,300,000
Lease commencement date		Dec. 15, 2017
Operating lease term	10 years	10 years
Operating lease term, optional extension	10 years	10 years
Accelerated depreciation expenses			$ 2,300,000
Security Deposit		$ 2,400,000
Repayment of tenant inducement allowance	$ 4,100,000
Rent expense			3,600,000	$ 2,000,000	$ 1,300,000
Contingency losses			0
Provisions for loss contingency			0
F. Hoffmann-La Roche Ltd. And Hoffmann-La Roche Inc. [Member]
Commitment And Contingencies [Line Items]
Upfront payments			2,000,000
Contingent payments			125,500,000
Royalty payments			1,000,000,000
Lease Agreements [Member]
Commitment And Contingencies [Line Items]
Tenant inducement			11,100,000
Repayment of tenant inducement allowance			$ 1,700,000
Lease Agreements [Member] | Maximum [Member]
Commitment And Contingencies [Line Items]
Tenant inducement	$ 27,900,000
Standby Letters of Credit [Member]
Commitment And Contingencies [Line Items]
Restricted cash		$ 900,000

Commitments and Contingencies_2

Commitments and Contingencies - Future Aggregate Minimum Lease Payments (Detail) $ in Thousands	Dec. 31, 2018USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2,019	$ 4,406
2,020	6,513
2,021	11,642
2,022	12,020
Thereafter	102,776
Total	$ 137,357

Net Loss per Share - Potentiall

Net Loss per Share - Potentially Dilutive Securities that were Not Included in Diluted Net Loss per Share Calculations (Detail) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	4,266,021	4,008,231	3,450,200
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	3,243,551	2,945,901	2,769,702
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	47,051	241,617	672,112
RSU [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	975,419	820,713
ESPP [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share			8,386

Selected Quarterly Financial _3

Selected Quarterly Financial Information (Unaudited) - Selected Consolidated Quarterly Financial Information (Unaudited) (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Loss from operations	$ (52,084)	$ (45,476)	$ (42,487)	$ (42,695)	$ (41,915)	$ (29,180)	$ (24,430)	$ (23,721)	$ (182,742)	$ (119,245)	$ (83,127)
Net loss	$ (49,201)	$ (43,068)	$ (40,368)	$ (41,556)	$ (41,252)	$ (28,557)	$ (23,883)	$ (23,332)	$ (174,193)	$ (117,024)	$ (82,468)
Basic and diluted net loss per common share	$ (0.93)	$ (0.83)	$ (0.78)	$ (0.87)	$ (0.95)	$ (0.66)	$ (0.55)	$ (0.60)	$ (3.41)	$ (2.76)	$ (2.48)