Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flow from operating activities:
Net loss	$ (37,470)	$ (17,820)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	6,981	3,631
Depreciation	461	300
Amortization of (discounts) premiums on investments	(292)	2
Change in operating assets and liabilities:
Receivable from collaboration partner		1,013
Prepaid expenses and other current assets	448	(444)
Operating lease right-of-use assets	635
Other long-term assets	(13)	(146)
Accounts payable	1,950	212
Accrued liabilities	(180)	(1,257)
Prepayment from collaboration partner	(9,874)	3,269
Other long-term liabilities	(692)	(43)
Deferred revenue		(5,331)
Net cash used in operating activities	(38,046)	(16,614)
Cash flow from investing activities:
Purchases of investments	(32,697)	(31,749)
Sales of investments	4,000
Maturities of investments	16,000	8,000
Purchases of property and equipment	(813)	(1,188)
Proceeds from sale of equipment		39
Net cash used in investing activities	(13,510)	(24,898)
Cash flow from financing activities:
Proceeds from issuance of common stock in follow-on offerings, net of issuance costs	271,520
Payments of financing related costs	(35)
Proceeds from exercise of stock options and employee stock purchase plan	280	806
Net cash provided by financing activities	271,765	806
Net increase (decrease) in cash, cash equivalents and restricted cash	220,209	(40,706)
Cash, cash equivalents and restricted cash, beginning of period	248,265	224,857
Cash, cash equivalents and restricted cash, end of period	468,474	184,151
Non-cash investing and financing activities:
Vesting of early exercised options and restricted stock	8	18
Unpaid financing-related costs	262
Unpaid portion of property and equipment purchases included in period-end accounts payable and accrued liabilities	$ 570	$ 317