Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash flow from operating activities:
Net loss	$ (276,213)	$ (67,698)	$ (57,010)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	32,994	19,341	6,138
Depreciation	1,948	1,567	1,294
Amortization of discount on investments	(1,619)	(279)	85
Loss on disposal of equipment			4
Change in operating assets and liabilities:
Receivable from collaboration partner		1,013	43,987
Prepaid expenses and other current assets	(2,612)	(2,884)	(482)
Operating lease right-of-use assets	2,618
Other long-term assets	290	(296)	(59)
Accounts payable	3,064	671	367
Accrued liabilities	11,665	8,945	2,968
Prepayment from collaboration partner	(12,973)	8,541	4,432
Operating lease liabilities	(2,838)
Other long-term liabilities	1,907	(178)	(120)
Deferred revenue		(33,558)	(11,442)
Net cash used in operating activities	(241,769)	(64,815)	(9,838)
Cash flow from investing activities:
Purchases of investments	(259,897)	(132,475)	(44,044)
Sales of investments	4,000	8,000	4,000
Maturities of investments	78,000	28,000	4,000
Purchases of property and equipment	(3,264)	(3,373)	(1,517)
Proceeds from sale of equipment			10
Net cash used in investing activities	(181,161)	(99,848)	(37,551)
Cash flow from financing activities:
Proceeds from issuance of common stock in follow-on offerings, net of issuance and financing costs	271,224	181,863	133,862
Proceeds from exercise of stock options and employee stock purchase plan	7,071	6,208	2,365
Payments of prior period offering costs			(38)
Net cash provided by financing activities	278,295	188,071	136,189
Net (decrease) increase in cash, cash equivalents and restricted cash	(144,635)	23,408	88,800
Cash, cash equivalents and restricted cash, beginning of period	248,265	224,857	136,057
Cash, cash equivalents and restricted cash, end of period	103,630	248,265	224,857
Non-cash investing and financing activities:
Vesting of early exercised options and restricted stock	15	53	184
Unpaid portion of property and equipment purchases included in period-end accounts payable and accrued liabilities	$ 9,756	$ 468	$ 283