Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flow from operating activities:
Net loss	$ (75,644)	$ (36,233)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	15,619	8,413
Depreciation	944	723
Amortization of discount on investments	(656)	(53)
Income tax benefit of unrealized gains on short and long-term investments	(218)
Change in operating assets and liabilities:
Receivable from collaboration partner		1,013
Prepaid expenses and other current assets	822	(728)
Operating lease right-of-use assets	1,279
Other long-term assets	125	(58)
Accounts payable	281	698
Accrued liabilities	4,428	1,388
Prepayment from collaboration partner	(10,717)	4,683
Other long-term liabilities	(1,390)	(88)
Deferred revenue		(11,970)
Net cash used in operating activities	(65,127)	(32,212)
Cash flow from investing activities:
Purchases of investments	(41,593)	(39,595)
Sales of investments	4,000
Maturities of investments	29,000	12,000
Purchases of property and equipment	(1,693)	(2,356)
Net cash used in investing activities	(10,286)	(29,951)
Cash flow from financing activities:
Proceeds from issuance of common stock in follow-on offerings, net of issuance and financing costs	271,224	182,069
Proceeds from exercise of stock options and employee stock purchase plan	1,837	2,350
Net cash provided by financing activities	273,061	184,419
Net increase in cash, cash equivalents and restricted cash	197,648	122,256
Cash, cash equivalents and restricted cash, beginning of period	248,265	224,857
Cash, cash equivalents and restricted cash, end of period	445,913	347,113
Non-cash investing and financing activities:
Vesting of early exercised options and restricted stock	15	32
Unpaid financing-related costs	11	206
Unpaid portion of property and equipment purchases included in period-end accounts payable and accrued liabilities	$ 17	$ 386