Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flow from operating activities:
Net loss	$ (69,868)	$ (37,470)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	10,752	6,981
Depreciation	779	461
Amortization of discount on investments	(418)	(292)
Loss on disposal of equipment	49
Change in operating assets and liabilities:
Prepaid expenses and other current assets	226	448
Operating lease right-of-use assets	1,066	635
Other long-term assets		(13)
Accounts payable	(467)	1,950
Accrued liabilities	(1,512)	(180)
Prepayment from collaboration partner		(9,874)
Operating lease liabilities	(504)
Other long-term liabilities		(692)
Net cash used in operating activities	(59,897)	(38,046)
Cash flow from investing activities:
Purchases of investments		(32,697)
Sales of investments	4,000	4,000
Maturities of investments	96,475	16,000
Purchases of property and equipment	(12,627)	(813)
Net cash provided by (used in) investing activities	87,848	(13,510)
Cash flow from financing activities:
Proceeds from issuance of common stock in follow-on offerings, net of issuance and financing costs		271,485
Proceeds from exercise of stock options and employee stock purchase plan	1,817	280
Net cash provided by financing activities	1,817	271,765
Net increase in cash, cash equivalents and restricted cash	29,768	220,209
Cash, cash equivalents and restricted cash, beginning of period	103,630	248,265
Cash, cash equivalents and restricted cash, end of period	133,398	468,474
Non-cash investing and financing activities:
Unpaid portion of property and equipment purchases included in period-end accounts payable and accrued liabilities	$ 2,015	570
Vesting of early exercised options and restricted stock		8
Unpaid financing-related costs		$ 262