Global Blood Therapeutics (GBT) 10-Q 9 May 19 2019 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2019	May 02, 2019
Document Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Mar. 31, 2019
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Entity Registrant Name	Global Blood Therapeutics, Inc.
Entity Central Index Key	0001629137
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Trading Symbol	GBT
Entity Emerging Growth Company	false
Entity Small Business	false
Entity Common Stock, Shares Outstanding		56,406,541

Condensed Consolidated Balance

Condensed Consolidated Balance Sheets - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 156,836	$ 275,357
Short-term marketable securities	300,431	202,177
Prepaid expenses and other current assets	8,222	8,246
Total current assets	465,489	485,780
Property and equipment, net	13,090	14,981
Long-term marketable securities	117,811	114,281
Operating lease right-of-use assets	14,075
Restricted cash	2,395	2,395
Other assets, noncurrent	200	206
Total assets	613,060	617,643
Current liabilities:
Accounts payable	7,346	6,046
Accrued liabilities	17,757	16,792
Accrued compensation	5,629	10,036
Other liabilities, current	1,359	899
Total current liabilities	32,091	33,773
Operating lease liabilities, noncurrent	24,401
Other liabilities, noncurrent	30	11,071
Total liabilities	56,522	44,844
Commitments and contingencies
Stockholders' equity:
Preferred stock, $0.001 par value, 5,000,000 shares authorized as of March 31, 2019 and December 31, 2018; no shares issued and outstanding
Common stock, $0.001 par value, 150,000,000 shares authorized as of March 31, 2019 (unaudited) and December 31, 2018, respectively; 56,337,045 and 55,640,299 shares issued and outstanding as of March 31, 2019 (unaudited) and December 31, 2018, respectively	56	56
Additional paid-in capital	1,076,979	1,044,941
Accumulated other comprehensive income (loss)	576	(48)
Accumulated deficit	(521,073)	(472,150)
Total stockholders' equity	556,538	572,799
Total liabilities and stockholders' equity	$ 613,060	$ 617,643

Condensed Consolidated Balanc_2

Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	Mar. 31, 2019	Dec. 31, 2018
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	56,337,045	55,640,299
Common stock, shares outstanding	56,337,045	55,640,299

Condensed Consolidated Statemen

Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Operating expenses:
Research and development	$ 34,468	$ 29,944
General and administrative	18,055	12,751
Total operating expenses	52,523	42,695
Loss from operations	(52,523)	(42,695)
Other income (expense):
Interest income, net	3,650	1,173
Other expenses, net	(50)	(34)
Total other income, net	3,600	1,139
Net loss	(48,923)	(41,556)
Other comprehensive income:
Net unrealized gain on marketable securities, net of tax	624	11
Comprehensive loss	$ (48,299)	$ (41,545)
Basic and diluted net loss per common share	$ (0.87)	$ (0.87)
Weighted-average number of shares used in computing basic and diluted net loss per common share	56,231,587	47,770,023

Condensed Consolidated Statem_2

Condensed Consolidated Statements of Stockholders' Equity - 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, 2017	$ 318,804	$ 46	$ 617,051	$ (336)	$ (297,957)
Beginning Balance, Shares at Dec. 31, 2017		46,131,723
Issuance of common stock upon equity offerings, net of issuance costs	255,067	$ 5	255,062
Issuance of common stock upon equity offerings, net of issuance costs, Shares		4,994,736
Issuance of common stock upon exercise of stock options	2,169	$ 1	2,168
Common stock issued on exercise of stock options, Shares		259,150
Issuance of common stock upon vesting of restricted share units, net of shares withheld for employee taxes	(5,404)		(5,404)
Issuance of common stock upon vesting of restricted share units, net of shares withheld for employee taxes, Shares		143,400
Issuance of common stock pursuant to ESPP purchases	784		784
Issuance of common stock pursuant to ESPP purchases, Shares		36,748
Vesting of restricted stock purchases	105		105
Vesting of restricted stock purchases, Shares		71,246
Stock-based compensation expense	7,784		7,784
Net unrealized gain on marketable securities	11			11
Net loss	(41,556)				(41,556)
Ending Balance at Mar. 31, 2018	537,764	$ 52	877,550	(325)	(339,513)
Ending Balance, Shares at Mar. 31, 2018		51,637,003
Beginning Balance at Dec. 31, 2018	572,799	$ 56	1,044,941	(48)	(472,150)
Beginning Balance, Shares at Dec. 31, 2018		55,640,299
Issuance of common stock upon equity offerings, net of issuance costs	21,246		21,246
Issuance of common stock upon equity offerings, net of issuance costs, Shares		511,363
Issuance of common stock upon exercise of stock options	$ 817		817
Common stock issued on exercise of stock options, Shares	52,288	52,288
Issuance of common stock upon vesting of restricted share units, net of shares withheld for employee taxes	$ (686)		(686)
Issuance of common stock upon vesting of restricted share units, net of shares withheld for employee taxes, Shares		78,155
Issuance of common stock pursuant to ESPP purchases	1,128		1,128
Issuance of common stock pursuant to ESPP purchases, Shares		30,745
Vesting of restricted stock purchases	80		80
Vesting of restricted stock purchases, Shares		24,195
Stock-based compensation expense	9,453		9,453
Net unrealized gain on marketable securities	624			624
Net loss	(48,923)				(48,923)
Ending Balance at Mar. 31, 2019	$ 556,538	$ 56	$ 1,076,979	$ 576	$ (521,073)
Ending Balance, Shares at Mar. 31, 2019		56,337,045

Condensed Consolidated Statem_3

Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
CASH FLOWS FROM OPERATING ACTIVITIES:
Net loss	$ (48,923)	$ (41,556)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,094	670
Amortization (accretion) of premium (discount) on marketable securities	(565)	93
Amortization of operating lease right-of-use assets	100
Stock-based compensation	9,453	7,784
Changes in operating assets and liabilities:
Prepaid expenses and other assets	49	(677)
Accounts payable	1,272	262
Accrued liabilities	887	125
Accrued compensation	(4,407)	(3,856)
Operating lease liabilities	(276)
Other liabilities		389
Net cash used in operating activities	(40,316)	(36,766)
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchase of property and equipment	(56)	(502)
Purchase of marketable securities	(164,177)
Maturities of marketable securities	63,582	42,076
Net cash provided by (used in) investing activities	(100,651)	41,574
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of common stock in public offering, net	21,207	254,940
Proceeds from issuance of common stock in settlement of employee stock purchase plan and exercise of stock options	1,925	2,821
Tax paid related to net share settlement of equity awards	(686)	(5,404)
Net cash provided by financing activities	22,446	252,357
Net increase (decrease) in cash, cash equivalents and restricted cash	(118,521)	257,165
Cash, cash equivalents and restricted cash at beginning of period	277,752	199,378
Cash, cash equivalents and restricted cash at end of period	159,231	456,543
SUPPLEMENTAL DISCLOSURES OF NON-CASH INVESTING AND FINANCING INFORMATION:
Accrued purchase of property and equipment	146	1,087
Accrued offering costs	$ (41)	$ 127

Organization and Basis of Prese

Organization and Basis of Presentation	3 Months Ended
	Mar. 31, 2019
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, Follow-on Offerings In December 2018, we completed a follow-on offering and issued 3,409,090 shares of common stock at a price of $44.00 per share with proceeds of $140.9 million net of underwriting costs and commissions, and estimated offering expenses. In addition, in January 2019, we sold an additional 511,363 shares of our common stock directly to the underwriters when they exercised their over-allotment option at the price of $44.00 per share for proceeds of $21.2 million net of underwriting costs and commissions. 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. Since inception through March 31, 2019, we have incurred cumulative net losses of $521.1 million. We expect to incur additional losses in the future to conduct product research and development and we recognize the need to raise additional capital to fully implement our business plan. We intend to raise such capital through the issuance of additional equity, and potentially through borrowings, and strategic alliances with partner companies. However, if such financing is not available at adequate levels, we will need to re-evaluate our operating plans. We believe that our existing cash and cash equivalents and marketable securities will be sufficient to fund our cash requirements for at least twelve months subsequent to the issuance of these financial statements.

Summary of Significant Accounti

Summary of Significant Accounting Policies	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Preparation The accompanying unaudited interim condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and applicable rules and regulations of the Securities and Exchange Commission (“SEC”) regarding interim financial reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP have been condensed or omitted, and accordingly the balance sheet as of December 31, 2018 has been derived from audited consolidated financial statements at that date but does not include all of the information required by U.S. GAAP for complete financial statements. These unaudited interim condensed consolidated financial statements have been prepared on the same basis as our annual consolidated financial statements and, in the opinion of management, reflect all adjustments (consisting only of normal recurring adjustments) that are necessary for a fair presentation of our financial information. The results of operations for the three months ended March 31, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019 or for any other interim period or for any other future year. The accompanying unaudited interim condensed consolidated financial statements and related financial information should be read in conjunction with the audited financial statements and the related notes thereto for the year ended December 31, 2018 included in our Annual Report on Form 10-K, filed with the SEC on February 27, 2019. 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 unaudited interim condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany transactions and balances have been eliminated upon consolidation. Significant Accounting Policies Except as noted below, there have been no material revisions in our significant accounting policies described in Note 2 to the consolidated financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2018. Leases Leases (Topic 842) Effective January 1, 2019 We determine if an arrangement is or contains a lease at inception by assessing whether the arrangement contains an identified asset and whether we have the right to control the identified asset. Right-of-use (“ROU”) assets represent our right to use an underlying asset for the lease term and lease liabilities represent our obligation to make lease payments arising from the lease. Lease liabilities are recognized at the lease commencement date based on the present value of future lease payments over the lease term. ROU assets are based on the measurement of the lease liability and also include any lease payments made prior to or on lease commencement and exclude lease incentives received and initial direct costs incurred, as applicable. As most of our leases do not provide an implicit rate, we use our incremental borrowing rate based on the information available at the commencement date in determining the present value of future payments. We give consideration to our credit risk, term of the lease, total lease payments and adjust for the impacts of collateral, as necessary, when calculating our incremental borrowing rates. The lease terms may include options to extend or terminate the lease when it is reasonably certain that we will exercise any such options. Lease cost for our operating leases is recognized on a straight-line basis over the lease term. We have elected to not separate lease and non-lease components for any leases within its existing classes of assets and, as a result, account for any lease and non-lease components as a single lease component. We have also elected to not recognize any leases within its existing classes of assets with a term of 12 months or less. Leases (Topic 840) Prior to the adoption of Topic 842 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. Accounting Pronouncements Adopted Leases (Topic 842) In February 2016 the Financial Accounting Standards Board issued Accounting Standards Update 2016-02, Leases (“Topic 842” or “ASU 2016-02”). ASU 2016-02 amends a number of aspects of lease accounting, including requiring lessees to recognize almost all leases with a term greater than one year as a ROU asset and corresponding liability, measured at the present value of the lease payments. On January 1, 2019, we adopted Topic 842 using the modified retrospective approach as of the adoption date. Results for the three months ended March 31, 2019 are presented under Topic 842. No prior period amounts were adjusted and continue to be reported in accordance with previous lease guidance, Accounting Standards Codification Topic 840, Leases (“Topic 840”). The new standard provides a number of optional practical expedients in transition. We elected the practical expedients to not reassess its prior conclusions about lease identification under the new standard, to not reassess lease classification, and to not reassess initial direct costs. We did not elect the practical expedient allowing the use-of-hindsight which would require us to reassess the lease term of its leases based on all facts and circumstances through the effective date and did not elect the practical expedient pertaining to land easements as this is not applicable to our current contract portfolio. The impact of our adoption of Topic 842 on the accompanying consolidated balance sheet as of January 1, 2019 was as follows (in thousands): December 31, 2018 Adjustments Due to January 1, 2019 Assets: Operating Lease ROU assets $ — $ 14,177 $ 14,177 Liabilities: Operating lease liabilities, current as included in other liabilities, current — 1,176 1,176 Deferred rent, current as included in other liabilities, current 712 (712 ) — Operating lease liabilities, noncurrent — 24,754 24,754 Deferred rent, noncurrent as included in other liabilities, noncurrent 11,041 (11,041 ) — The adjustments due to our adoption of Topic 842 related to the recognition of operating lease ROU assets and operating lease liabilities for the existing operating leases. A cumulative-effect adjustment to beginning retained earnings was not required.

Fair Value Measurements

Fair Value Measurements	3 Months Ended
	Mar. 31, 2019
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, 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 condensed consolidated balance sheets. The remaining financial instruments are reported on our condensed 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 condensed 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 – Inputs are observable, unadjusted quoted prices in active markets for similar assets or liabilities, unadjusted quoted prices for identical or similar assets or liabilities in markets that are not active, or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the related assets or liabilities; and Level 3 – Unobservable inputs that are significant to the measurement of the fair value of the assets or liabilities that are supported by little or no market data. The following table summarizes our financial assets measured at fair value on a recurring basis (in thousands): March 31, 2019 Total Level 1 Level 2 Level 3 Financial Assets: Money market funds $ 145,413 $ 145,413 $ — $ — Corporate debt securities 143,927 — 143,927 — U.S. government agency securities 109,458 — 109,458 — Certificates of deposits 6,957 — 6,957 — U.S. government securities 169,231 — 169,231 — Total financial assets $ 574,986 $ 145,413 $ 429,573 $ — 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 $ — 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 markets, benchmark securities, bids, offers and reference data including market research publications. At March 31, 2019 and December 31, 2018, the weighted average remaining contractual maturities of our Level 2 investments was less than one year and all of these investments are rated A-1/P-1/F1 or A/A2, or higher by Moody’s and S&P. There were no transfers between Level 1 and Level 2 during the periods presented.

Available-for-Sale Securities

Available-for-Sale Securities	3 Months Ended
	Mar. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Available-for-Sale Securities	4. Available-for-Sale Securities Estimated fair values of available-for-sale securities are generally based on prices obtained from commercial pricing services. The following table is a summary of available-for-sale securities recorded in cash and cash equivalents, restricted cash, or marketable securities in our condensed consolidated balance sheets (in thousands): March 31, 2019 December 31, 2018 Amortized Cost Unrealized Gains Unrealized (Losses) Estimated Fair Value Amortized Cost Unrealized Gains Unrealized (Losses) Estimated Fair Value Financial Assets: Money market funds $ 145,413 $ — $ — $ 145,413 $ 275,234 $ — $ — $ 275,234 Corporate debt securities 143,664 278 (15 ) 143,927 110,053 69 (95 ) 110,027 U.S. government agency securities 109,364 98 (4 ) 109,458 88,042 40 (54 ) 88,028 Certificates of deposits 6,926 32 (1 ) 6,957 6,681 1 (7 ) 6,675 U.S. government securities 169,043 194 (6 ) 169,231 111,730 60 (62 ) 111,728 Total $ 574,410 $ 602 $ (26 ) $ 574,986 $ 591,740 $ 170 $ (218 ) $ 591,692 The following table summarizes the classification of the available-for-sale securities on our condensed consolidated balance sheets (in thousands): March 31, 2019 December 31, 2018 Cash and cash equivalents $ 156,744 $ 275,234 Short-term marketable securities 300,431 202,177 Long-term marketable securities 117,811 114,281 Total $ 574,986 $ 591,692 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 were temporary in nature during the periods presented.

Balance Sheet Components

Balance Sheet Components	3 Months Ended
	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Balance Sheet Components	5. Balance Sheet Components Property and Equipment Property and equipment consists of the following (in thousands): March 31, 2019 December 31, 2018 Laboratory equipment $ 7,551 $ 7,363 Computer equipment 1,501 1,501 Leasehold improvements 13,785 13,785 Construction-in-progress 253 239 Total property and equipment 23,090 22,888 Less: accumulated depreciation and amortization (10,000 ) (7,907 ) Property and equipment, net $ 13,090 $ 14,981 Accrued liabilities Accrued liabilities consist of the following (in thousands): March 31, 2019 December 31, 2018 Accrued clinical and manufacturing expenses $ 15,126 $ 15,121 Accrued professional and consulting services 2,104 1,016 Other 527 655 Total accrued liabilities $ 17,757 $ 16,792 Other liabilities, current and noncurrent Other liabilities consist of the following (in thousands): March 31, 2019 December 31, 2018 Operating lease liabilities, current $ 1,252 $ — Restricted shares subject to repurchase, current 77 157 Deferred rent, current — 712 Other payable 30 30 Total other liabilities, current $ 1,359 $ 899 Deferred rent, noncurrent $ — $ 11,041 Other liabilities, noncurrent 30 30 Total other liabilities, noncurrent $ 30 $ 11,071

Stock Based Compensation

Stock Based Compensation	3 Months Ended
	Mar. 31, 2019
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Stock Based Compensation	6. Stock-Based Compensation We have three stock-based compensation plans – the Amended and Restated 2017 Inducement Equity Plan (the “2017 Inducement Plan”), the 2015 Stock Option and Incentive Plan (the “2015 Plan”) and the 2012 Stock Option and Grant Plan (the “2012 Plan”). As of March 31, 2019, there were 243,325 shares reserved under the 2017 Inducement Plan and 3,630,786 shares reserved under the 2015 Plan for the future issuance of equity awards. Upon adoption of the 2015 Plan in July 2015, no new awards or grants are permitted under the 2012 Plan. See Note 7 to the Consolidated Financial Statements included in our Annual Report on Form 10-K for the year ended December 31, 2018 for additional information related to these stock-based compensation plans. Stock Options The following summarizes option activity under the 2017 Inducement Plan, 2015 Plan and 2012 Plan: Number of Options Weighted- Average Exercise Price Outstanding — December 31, 2018 3,243,551 $ 29.74 Options granted 685,765 48.93 Options exercised (52,288 ) 15.62 Options canceled (63,033 ) 37.22 Outstanding — March 31, 2019 3,813,995 $ 33.26 The fair values of stock options granted to employees were calculated using the following assumptions: Three Months Ended 2019 2018 Expected term (in years) 6.1 6.1 Volatility 71.6%-72.2% 69.1%-69.3% Risk-free interest rate 2.5%-2.6% 2.6%-2.7% Dividend yield — — Restricted Stock Units 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: Number of RSUs Weighted- Average Grant Date Fair Value Non-vested units — December 31, 2018 816,169 $ 43.34 RSUs granted 772,050 48.90 RSUs vested (92,359 ) 41.16 RSUs forfeited (28,983 ) 45.94 Non-vested units — March 31, 2019 1,466,877 $ 46.36 Stock-Based Compensation Expense Total stock-based compensation recognized by function was as follows (in thousands): Three Months Ended March 31, 2019 2018 Research and development $ 4,023 $ 2,968 General and administrative 5,430 4,816 Total stock-based compensation expense $ 9,453 $ 7,784

Net Loss per Share

Net Loss per Share	3 Months Ended
	Mar. 31, 2019
Earnings Per Share [Abstract]
Net Loss per Share	7. 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: Three Months Ended March 31, 2019 2018 Options to purchase common stock 3,813,995 3,379,550 Restricted stock subject to future vesting 22,856 1,040,806 Restricted stock units 1,626,127 170,371 Total 5,462,978 4,590,727

Commitments and Contingencies

Commitments and Contingencies	3 Months Ended
	Mar. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	8. Commitments and Contingencies Leases We have operating leases for our headquarters, where we have office and research and development laboratory facilities, and equipment. Our leases have remaining lease terms of 1 to 10 For the three months ended March 31, 2019, we incurred $1.9 million of lease costs included in operating expenses in the consolidated statement of operations in relation to these operating leases, of which $0.9 million was variable lease cost and not included within the measurement of our operating ROU assets and operating lease liabilities. The variable lease cost is comprised primarily of our cost in certain research and development arrangement that contains embedded equipment, and our proportionate share of operating expenses, property taxes, and insurance in relation with our facility lease. These costs are classified as operating lease expense due to our election to not separate lease and non-lease components. Cash paid for amounts included in the measurement of operating lease liabilities for the three months ended March 31, 2019 was $1.1 million and was included in net cash used in operating activities in our consolidated statements of cash flows. We assumed $25.9 million of operating lease liabilities arising from obtaining the ROU assets for the three months ended March 31, 2019. The majority of our lease costs are driven by our operating lease for our headquarters in South San Francisco, where we have office and research and development laboratory 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 period. Future minimum rental payments under the Lease during the 10-year term are $48.3 million in the aggregate. The Lease further provides that we are obligated to pay to the landlord certain costs, including taxes and operating expenses. The Lease term commenced in November 2017 as we gained control over physical access to the Existing Premises. We have acquired $11.1 million of leasehold improvements at our Existing Premises with the tenant inducement allowance provided under the Lease. We are required to repay $1.7 million of the tenant inducement allowance to the landlord in the form of additional monthly rent with interest applied over the term of the Lease. 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 consisting of approximately 164,150 rentable square feet of space (the “Substitute Premises”) when the Substitute Premises are ready for occupancy (the “Substitute Premises Commencement Date”). The Lease Amendment has a contractual term (the “Substitute Premises Term”) of 10 years from the Substitute Premises Commencement Date. The Lease Amendment grants us an option to extend the Lease for an additional 10-year period. Future minimum rental payments under the Lease Amendment during the 10-year term are $121.5 million in the aggregate. Under the Lease Amendment, we are obligated to pay to the landlord certain costs, including taxes and operating expenses. The Lease Amendment also provides a tenant inducement allowance of up to $27.9 million, of which $4.1 million, if utilized, would be repaid to the landlord in the form of additional monthly rent with interest applied. We intend to vacate the Existing Premises and surrender and deliver the Existing Premises to the 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 the remaining useful life of the leasehold improvements at our Existing Premises and started to amortize the leasehold improvements over the remaining period of expected use, resulting in an acceleration of depreciation expenses for approximately $1.7 million. As of March 31, 2019, the maturities of our operating lease liabilities were as follows (in thousands): Year ending December 31, Amount (a) 2019 (nine months) $ 3,364 2020 4,576 2021 4,701 2022 4,856 2023 5,017 2024 5,183 Thereafter 16,114 Total lease payments 43,811 Less: Imputed interest (b) (18,158 ) Present value of operating lease liabilities $ 25,653 (a) Operating lease payments exclude $121.5 million of legally binding minimum lease payments for Lease Amendment, which has been signed by us but has not yet commenced. (b) Imputed interest is calculated using the interest rate for each lease. As of March 31, 2019, the weighted average remaining lease term is 8.7 years and the weighted average discount rate used to determine the operating lease liability was 13.2%. Future annual minimum lease payments due under the Lease and Lease Amendment as of December 31, 2018 were as follows (in thousands): Year ending December 31, Amount (c) 2019 $ 4,406 2020 6,513 2021 11,642 2022 12,020 2023 12,409 Thereafter 90,367 Total $ 137,357 (c) The table above is prepared under the assumption that the Substitute Premises Commencement at the Substitute Premises starts on June 30, 2020. Rent expense for the three months ended March 31, 2019 and 2018 was $0.9 million and $0.9 million, respectively. 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.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Basis of Presentation	Basis of Preparation The accompanying unaudited interim condensed consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (“U.S. GAAP”) and applicable rules and regulations of the Securities and Exchange Commission (“SEC”) regarding interim financial reporting. As permitted under those rules, certain footnotes or other financial information that are normally required by U.S. GAAP have been condensed or omitted, and accordingly the balance sheet as of December 31, 2018 has been derived from audited consolidated financial statements at that date but does not include all of the information required by U.S. GAAP for complete financial statements. These unaudited interim condensed consolidated financial statements have been prepared on the same basis as our annual consolidated financial statements and, in the opinion of management, reflect all adjustments (consisting only of normal recurring adjustments) that are necessary for a fair presentation of our financial information. The results of operations for the three months ended March 31, 2019 are not necessarily indicative of the results to be expected for the year ending December 31, 2019 or for any other interim period or for any other future year. The accompanying unaudited interim condensed consolidated financial statements and related financial information should be read in conjunction with the audited financial statements and the related notes thereto for the year ended December 31, 2018 included in our Annual Report on Form 10-K, filed with the SEC on February 27, 2019.
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 unaudited interim condensed consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All intercompany transactions and balances have been eliminated upon consolidation.
Significant Accounting Policies	Significant Accounting Policies Except as noted below, there have been no material revisions in our significant accounting policies described in Note 2 to the consolidated financial statements included in our Annual Report on Form 10-K for the year ended December 31, 2018.
Leases	Leases Leases (Topic 842) Effective January 1, 2019 We determine if an arrangement is or contains a lease at inception by assessing whether the arrangement contains an identified asset and whether we have the right to control the identified asset. Right-of-use (“ROU”) assets represent our right to use an underlying asset for the lease term and lease liabilities represent our obligation to make lease payments arising from the lease. Lease liabilities are recognized at the lease commencement date based on the present value of future lease payments over the lease term. ROU assets are based on the measurement of the lease liability and also include any lease payments made prior to or on lease commencement and exclude lease incentives received and initial direct costs incurred, as applicable. As most of our leases do not provide an implicit rate, we use our incremental borrowing rate based on the information available at the commencement date in determining the present value of future payments. We give consideration to our credit risk, term of the lease, total lease payments and adjust for the impacts of collateral, as necessary, when calculating our incremental borrowing rates. The lease terms may include options to extend or terminate the lease when it is reasonably certain that we will exercise any such options. Lease cost for our operating leases is recognized on a straight-line basis over the lease term. We have elected to not separate lease and non-lease components for any leases within its existing classes of assets and, as a result, account for any lease and non-lease components as a single lease component. We have also elected to not recognize any leases within its existing classes of assets with a term of 12 months or less. Leases (Topic 840) Prior to the adoption of Topic 842 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.
Accounting Pronouncements Adopted	Accounting Pronouncements Adopted Leases (Topic 842) In February 2016 the Financial Accounting Standards Board issued Accounting Standards Update 2016-02, Leases (“Topic 842” or “ASU 2016-02”). ASU 2016-02 amends a number of aspects of lease accounting, including requiring lessees to recognize almost all leases with a term greater than one year as a ROU asset and corresponding liability, measured at the present value of the lease payments. On January 1, 2019, we adopted Topic 842 using the modified retrospective approach as of the adoption date. Results for the three months ended March 31, 2019 are presented under Topic 842. No prior period amounts were adjusted and continue to be reported in accordance with previous lease guidance, Accounting Standards Codification Topic 840, Leases (“Topic 840”). The new standard provides a number of optional practical expedients in transition. We elected the practical expedients to not reassess its prior conclusions about lease identification under the new standard, to not reassess lease classification, and to not reassess initial direct costs. We did not elect the practical expedient allowing the use-of-hindsight which would require us to reassess the lease term of its leases based on all facts and circumstances through the effective date and did not elect the practical expedient pertaining to land easements as this is not applicable to our current contract portfolio. The impact of our adoption of Topic 842 on the accompanying consolidated balance sheet as of January 1, 2019 was as follows (in thousands): December 31, 2018 Adjustments Due to January 1, 2019 Assets: Operating Lease ROU assets $ — $ 14,177 $ 14,177 Liabilities: Operating lease liabilities, current as included in other liabilities, current — 1,176 1,176 Deferred rent, current as included in other liabilities, current 712 (712 ) — Operating lease liabilities, noncurrent — 24,754 24,754 Deferred rent, noncurrent as included in other liabilities, noncurrent 11,041 (11,041 ) — The adjustments due to our adoption of Topic 842 related to the recognition of operating lease ROU assets and operating lease liabilities for the existing operating leases. A cumulative-effect adjustment to beginning retained earnings was not required.

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	3 Months Ended
	Mar. 31, 2019
Accounting Policies [Abstract]
Schedule of Adoption of Topic 842 on Consolidated balance sheet	The impact of our adoption of Topic 842 on the accompanying consolidated balance sheet as of January 1, 2019 was as follows (in thousands): December 31, 2018 Adjustments Due to January 1, 2019 Assets: Operating Lease ROU assets $ — $ 14,177 $ 14,177 Liabilities: Operating lease liabilities, current as included in other liabilities, current — 1,176 1,176 Deferred rent, current as included in other liabilities, current 712 (712 ) — Operating lease liabilities, noncurrent — 24,754 24,754 Deferred rent, noncurrent as included in other liabilities, noncurrent 11,041 (11,041 ) —

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	3 Months Ended
	Mar. 31, 2019
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): March 31, 2019 Total Level 1 Level 2 Level 3 Financial Assets: Money market funds $ 145,413 $ 145,413 $ — $ — Corporate debt securities 143,927 — 143,927 — U.S. government agency securities 109,458 — 109,458 — Certificates of deposits 6,957 — 6,957 — U.S. government securities 169,231 — 169,231 — Total financial assets $ 574,986 $ 145,413 $ 429,573 $ — 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 $ —

Available-for-Sale Securities (

Available-for-Sale Securities (Tables)	3 Months Ended
	Mar. 31, 2019
Investments, Debt and Equity Securities [Abstract]
Summary of Available-for-Sale Securities	The following table is a summary of available-for-sale securities recorded in cash and cash equivalents, restricted cash, or marketable securities in our condensed consolidated balance sheets (in thousands): March 31, 2019 December 31, 2018 Amortized Cost Unrealized Gains Unrealized (Losses) Estimated Fair Value Amortized Cost Unrealized Gains Unrealized (Losses) Estimated Fair Value Financial Assets: Money market funds $ 145,413 $ — $ — $ 145,413 $ 275,234 $ — $ — $ 275,234 Corporate debt securities 143,664 278 (15 ) 143,927 110,053 69 (95 ) 110,027 U.S. government agency securities 109,364 98 (4 ) 109,458 88,042 40 (54 ) 88,028 Certificates of deposits 6,926 32 (1 ) 6,957 6,681 1 (7 ) 6,675 U.S. government securities 169,043 194 (6 ) 169,231 111,730 60 (62 ) 111,728 Total $ 574,410 $ 602 $ (26 ) $ 574,986 $ 591,740 $ 170 $ (218 ) $ 591,692
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 condensed consolidated balance sheets (in thousands): March 31, 2019 December 31, 2018 Cash and cash equivalents $ 156,744 $ 275,234 Short-term marketable securities 300,431 202,177 Long-term marketable securities 117,811 114,281 Total $ 574,986 $ 591,692

Balance Sheet Components (Table

Balance Sheet Components (Tables)	3 Months Ended
	Mar. 31, 2019
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Property and Equipment	Property and equipment consists of the following (in thousands): March 31, 2019 December 31, 2018 Laboratory equipment $ 7,551 $ 7,363 Computer equipment 1,501 1,501 Leasehold improvements 13,785 13,785 Construction-in-progress 253 239 Total property and equipment 23,090 22,888 Less: accumulated depreciation and amortization (10,000 ) (7,907 ) Property and equipment, net $ 13,090 $ 14,981
Accrued Liabilities	Accrued liabilities consist of the following (in thousands): March 31, 2019 December 31, 2018 Accrued clinical and manufacturing expenses $ 15,126 $ 15,121 Accrued professional and consulting services 2,104 1,016 Other 527 655 Total accrued liabilities $ 17,757 $ 16,792
Other Liabilities	Other liabilities consist of the following (in thousands): March 31, 2019 December 31, 2018 Operating lease liabilities, current $ 1,252 $ — Restricted shares subject to repurchase, current 77 157 Deferred rent, current — 712 Other payable 30 30 Total other liabilities, current $ 1,359 $ 899 Deferred rent, noncurrent $ — $ 11,041 Other liabilities, noncurrent 30 30 Total other liabilities, noncurrent $ 30 $ 11,071

Stock Based Compensation (Table

Stock Based Compensation (Tables)	3 Months Ended
	Mar. 31, 2019
Stock Option Activity	The following summarizes option activity under the 2017 Inducement Plan, 2015 Plan and 2012 Plan: Number of Options Weighted- Average Exercise Price Outstanding — December 31, 2018 3,243,551 $ 29.74 Options granted 685,765 48.93 Options exercised (52,288 ) 15.62 Options canceled (63,033 ) 37.22 Outstanding — March 31, 2019 3,813,995 $ 33.26
Valuation Assumptions for Stock Awards	The fair values of stock options granted to employees were calculated using the following assumptions: Three Months Ended 2019 2018 Expected term (in years) 6.1 6.1 Volatility 71.6%-72.2% 69.1%-69.3% Risk-free interest rate 2.5%-2.6% 2.6%-2.7% Dividend yield — —
Stock-based Compensation Expense Recognized	Total stock-based compensation recognized by function was as follows (in thousands): Three Months Ended March 31, 2019 2018 Research and development $ 4,023 $ 2,968 General and administrative 5,430 4,816 Total stock-based compensation expense $ 9,453 $ 7,784
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: Number of RSUs Weighted- Average Grant Date Fair Value Non-vested units — December 31, 2018 816,169 $ 43.34 RSUs granted 772,050 48.90 RSUs vested (92,359 ) 41.16 RSUs forfeited (28,983 ) 45.94 Non-vested units — March 31, 2019 1,466,877 $ 46.36

Net Loss per Share (Tables)

Net Loss per Share (Tables)	3 Months Ended
	Mar. 31, 2019
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: Three Months Ended March 31, 2019 2018 Options to purchase common stock 3,813,995 3,379,550 Restricted stock subject to future vesting 22,856 1,040,806 Restricted stock units 1,626,127 170,371 Total 5,462,978 4,590,727

Commitments and Contingencies (

Commitments and Contingencies (Tables)	3 Months Ended
	Mar. 31, 2019
Commitments and Contingencies Disclosure [Abstract]
Schedule of Lessee, Operating Lease, Liability, Maturity	As of March 31, 2019, the maturities of our operating lease liabilities were as follows (in thousands): Year ending December 31, Amount (a) 2019 (nine months) $ 3,364 2020 4,576 2021 4,701 2022 4,856 2023 5,017 2024 5,183 Thereafter 16,114 Total lease payments 43,811 Less: Imputed interest (b) (18,158 ) Present value of operating lease liabilities $ 25,653 (a) Operating lease payments exclude $121.5 million of legally binding minimum lease payments for Lease Amendment, which has been signed by us but has not yet commenced. (b) Imputed interest is calculated using the interest rate for each lease.
Schedule of Future Minimum Rental Payments for Operating Leases	Future annual minimum lease payments due under the Lease and Lease Amendment as of December 31, 2018 were as follows (in thousands): Year ending December 31, Amount (c) 2019 $ 4,406 2020 6,513 2021 11,642 2022 12,020 2023 12,409 Thereafter 90,367 Total $ 137,357 (c) The table above is prepared under the assumption that the Substitute Premises Commencement at the Substitute Premises starts on June 30, 2020.

Organization and Basis of Pre_2

Organization and Basis of Presentation - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended
	Jan. 31, 2019	Dec. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Class of Stock [Line Items]
Proceeds from issuance of common stock, net of issuance costs			$ 21,207	$ 254,940
Cumulative net losses		$ (472,150)	$ (521,073)
Common Stock [Member]
Class of Stock [Line Items]
Common stock shares sold		3,409,090	511,363	4,994,736
Share price per share | $ / shares		$ 44
Proceeds from issuance of common stock, net of issuance costs		$ 140,900
Common Stock [Member] | Over-Allotment Option [Member]
Class of Stock [Line Items]
Common stock shares sold	511,363
Share price per share | $ / shares	$ 44
Proceeds from issuance of common stock, net of issuance costs	$ 21,200

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Schedule of Adoption Of Topic 842 on Consolidated balance Sheet (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Assets:
Operating Lease ROU assets	$ 14,075
Liabilities:
Operating lease liabilities, current as included in other liabilities, current	1,252
Deferred rent, current as included in other liabilities, current			$ 712
Operating lease liabilities, noncurrent	$ 24,401
Deferred rent, noncurrent as included in other liabilities, noncurrent			11,041
Adjustment Due To Adoption Of Topic 842 [Member]
Assets:
Operating Lease ROU assets			14,177
Liabilities:
Operating lease liabilities, current as included in other liabilities, current			1,176
Deferred rent, current as included in other liabilities, current			(712)
Operating lease liabilities, noncurrent			24,754
Deferred rent, noncurrent as included in other liabilities, noncurrent			$ (11,041)
Restatement Adjustment [Member]
Assets:
Operating Lease ROU assets		$ 14,177
Liabilities:
Operating lease liabilities, current as included in other liabilities, current		1,176
Operating lease liabilities, noncurrent		$ 24,754

Fair Value Measurements - Fair

Fair Value Measurements - Fair Value Assets Measured on Recurring Basis (Detail) - Fair Value Measurements, Recurring [Member] - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	$ 574,986	$ 591,692
U.S. Government Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	169,231	111,728
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	145,413	275,234
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	429,573	316,458
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	169,231	111,728
Money Market Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	145,413	275,234
Money Market Funds [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	145,413	275,234
Corporate Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	143,927	110,027
Corporate Debt Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	143,927	110,027
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	109,458	88,028
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	109,458	88,028
Certificates of Deposits [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	6,957	6,675
Certificates of Deposits [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Financial assets, fair value	$ 6,957	$ 6,675

Available-for-Sale Securities -

Available-for-Sale Securities - Summary of Available-for-Sale Securities (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Available for sale securities, Amortized Cost	$ 574,410	$ 591,740
Available for sale securities, Unrealized Gains	602	170
Available for sale securities, Unrealized (Losses)	(26)	(218)
Available for sale securities, Estimated Fair Value	574,986	591,692
Corporate Debt Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available for sale securities, Amortized Cost	143,664	110,053
Available for sale securities, Unrealized Gains	278	69
Available for sale securities, Unrealized (Losses)	(15)	(95)
Available for sale securities, Estimated Fair Value	143,927	110,027
U.S. Government Agency Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available for sale securities, Amortized Cost	109,364	88,042
Available for sale securities, Unrealized Gains	98	40
Available for sale securities, Unrealized (Losses)	(4)	(54)
Available for sale securities, Estimated Fair Value	109,458	88,028
Money Market Funds [Member]
Debt Securities, Available-for-sale [Line Items]
Available for sale securities, Amortized Cost	145,413	275,234
Available for sale securities, Estimated Fair Value	145,413	275,234
Certificates of Deposits [Member]
Debt Securities, Available-for-sale [Line Items]
Available for sale securities, Amortized Cost	6,926	6,681
Available for sale securities, Unrealized Gains	32	1
Available for sale securities, Unrealized (Losses)	(1)	(7)
Available for sale securities, Estimated Fair Value	6,957	6,675
U.S. Government Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Available for sale securities, Amortized Cost	169,043	111,730
Available for sale securities, Unrealized Gains	194	60
Available for sale securities, Unrealized (Losses)	(6)	(62)
Available for sale securities, Estimated Fair Value	$ 169,231	$ 111,728

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	Mar. 31, 2019	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Cash and cash equivalents	$ 156,744	$ 275,234
Short-term marketable securities	300,431	202,177
Long-term marketable securities	117,811	114,281
Total	$ 574,986	$ 591,692

Balance Sheet Components - Prop

Balance Sheet Components - Property and Equipment (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 23,090	$ 22,888
Less: accumulated depreciation and amortization	(10,000)	(7,907)
Property and equipment, net	13,090	14,981
Laboratory Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	7,551	7,363
Computer Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	1,501	1,501
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	13,785	13,785
Construction-in-Progress [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 253	$ 239

Balance Sheet Components - Accr

Balance Sheet Components - Accrued Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Balance Sheet Components [Abstract]
Accrued clinical and manufacturing expenses	$ 15,126	$ 15,121
Accrued professional and consulting services	2,104	1,016
Other	527	655
Total accrued liabilities	$ 17,757	$ 16,792

Balance Sheet Components - Othe

Balance Sheet Components - Other Liabilities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Balance Sheet Components [Abstract]
Operating lease liabilities, current	$ 1,252
Restricted shares subject to repurchase,current	77	$ 157
Deferred rent, current		712
Other payable	30	30
Total other liabilities, current	1,359	899
Deferred rent, noncurrent		11,041
Other liabilities, noncurrent	30	30
Total other liabilities, noncurrent	$ 30	$ 11,071

Stock Based Compensation - Plan

Stock Based Compensation - Plan Summary - Additional Information (Detail)	Mar. 31, 2019shares
2017 Inducement Equity Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance | shares	243,325
2015 Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized for issuance | shares	3,630,786

Stock Based Compensation - Stoc

Stock Based Compensation - Stock Option Activity (Detail)	3 Months Ended
	Mar. 31, 2019$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Beginning balance, outstanding, Number of Options | shares	3,243,551
Options granted, Number of Options | shares	685,765
Options exercised, Number of Options | shares	(52,288)
Options canceled, Number of Options | shares	(63,033)
Ending balance, outstanding, Number of Options | shares	3,813,995
Weighted- Average Exercise Price
Beginning balance, Outstanding, Weighted-Average Exercise Price | $ / shares	$ 29.74
Options granted, Weighted-Average Exercise Price | $ / shares	48.93
Options exercised, Weighted-Average Exercise Price | $ / shares	15.62
Options canceled, Weighted-Average Exercise Price | $ / shares	37.22
Ending balance, Outstanding, Weighted-Average Exercise Price | $ / shares	$ 33.26

Stock Based Compensation - Valu

Stock Based Compensation - Valuation Assumptions for Stock Options Granted (Detail) - Stock Options [Member]	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
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
Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility, minimum	71.60%	69.10%
Risk-free interest rate, minimum	2.50%	2.60%
Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility, maximum	72.20%	69.30%
Risk-free interest rate, maximum	2.60%	2.70%

Stock Based Compensation - Summ

Stock Based Compensation - Summary of RSU Activity Granted to Employees with Service-Based Vesting (Detail) - 2017 Inducement Plan and 2015 Plan [Member] - RSU [Member]	3 Months Ended
	Mar. 31, 2019$ / sharesshares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Beginning Balance - Non-vested market-condition awards | shares	816,169
RSUs granted, Number of RSUs | shares	772,050
RSUs vested, Number of RSUs | shares	(92,359)
RSUs forfeited, Number of RSUs | shares	(28,983)
Ending Balance - Non-vested market-condition awards | shares	1,466,877
Beginning Balance - Non-vested units, Weighted-Average Grant Date Fair Value | $ / shares	$ 43.34
RSUs granted, Weighted-Average Grant Date Fair Value | $ / shares	48.90
RSUs vested, Weighted-Average Grant Date Fair Value | $ / shares	41.16
RSUs forfeited, Weighted-Average Grant Date Fair Value | $ / shares	45.94
Ending Balance - Non-vested units, Weighted-Average Grant Date Fair Value | $ / shares	$ 46.36

Stock Based Compensation - St_2

Stock Based Compensation - Stock-based Compensation Expense Recognized (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 9,453	$ 7,784
Research and Development Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	4,023	2,968
General and Administrative Expense [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Compensation Cost [Line Items]
Total stock-based compensation expense	$ 5,430	$ 4,816

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	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	5,462,978	4,590,727
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	3,813,995	3,379,550
Restricted Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	22,856	1,040,806
RSU [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Antidilutive securities excluded from earnings per share	1,626,127	170,371

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) $ in Thousands	1 Months Ended		3 Months Ended
	Aug. 31, 2018USD ($)ft²	Mar. 31, 2017USD ($)ft²	Mar. 31, 2019USD ($)	Mar. 31, 2018USD ($)
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	$ 48,300
Lease commencement date		Dec. 15, 2017
Operating lease term	10 years	10 years
Operating lease term, optional extension	10 years	10 years
Repayment of tenant inducement allowance	$ 4,100
Lease term			8 years 8 months 12 days
Operating Lease Cost			$ 1,900
variable lease cost			900
Cash paid for amounts included in the measurement of operating lease liabilities			$ 1,100
leasehold improvements Acquired		$ 11,100
Repayemnt of tenant inducement allowance to the landlord		1,700
acceleration of depreciation expenses		$ 1,700
weighted average discount rate			13.20%
Rent expense			$ 900	$ 900
Operating liabilities			$ 25,653
Maximum [Member]
Commitment And Contingencies [Line Items]
Lease term			10 years
Minimum [Member]
Commitment And Contingencies [Line Items]
Lease term			1 year
Lease Agreements [Member] | Maximum [Member]
Commitment And Contingencies [Line Items]
Tenant inducement	$ 27,900

Commitments and Contingencies_2

Commitments and Contingencies - Lessee Operating Lease Liability Maturity (Detail) $ in Thousands	Mar. 31, 2019USD ($)
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019 (nine months)	$ 3,364
2020	4,576
2021	4,701
2022	4,856
2023	5,017
2024	5,183
Thereafter	16,114
Total lease payments	43,811
Less: Imputed interest	(18,158)
Present value of operating lease liabilities	$ 25,653

Commitments and Contingencies_3

Commitments and Contingencies - Lessee Operating Lease Liability Maturity (Detail) (Parenthetical) $ in Millions	Mar. 31, 2019USD ($)
Commitments and Contingencies Disclosure [Abstract]
Lessee Operating Lease Lease Not Yet Commenced Legally Binding Minimum Lease Payments	$ 121.5

Commitments and Contingencies_4

Commitments and Contingencies - Minimum Lease payments (Detail) $ in Thousands	Dec. 31, 2018USD ($)
Commitments and Contingencies Disclosure [Abstract]
2019	$ 4,406
2020	6,513
2021	11,642
2022	12,020
2023	12,409
Thereafter	90,367
Total	$ 137,357