Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Operating activities
Net loss	$ (340,141)	$ (306,620)	$ (290,976)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	53,865	51,351	51,696
Depreciation and amortization expense	9,345	8,332	7,070
Non-cash operating lease expense	1,948	1,457	964
Amortization (accretion) of investment premiums (discounts)	1,769	828	(1,330)
Loss on disposals of property and equipment	21	130	1,027
Asset retirement obligation accretion expense	87	78	71
Changes in operating assets and liabilities:
Accounts receivable	264	(1,250)	0
Prepaid expenses and other current assets	8,182	(8,666)	(998)
Operating lease assets	0	886	239
Other assets	(1,727)	(219)	322
Accounts payable	9,067	(815)	4,213
Accrued compensation	4,692	5,752	4,070
Accrued research and development expenses	(2,362)	7,472	(10,869)
Other current liabilities	1,618	(187)	(394)
Deferred revenue	35,218	61,250	0
Operating lease liabilities	(1,859)	(1,316)	(731)
Other long-term liabilities	(509)	778	0
Net cash used in operating activities	(220,522)	(180,759)	(235,626)
Investing activities
Purchases of short-term investments	(301,129)	(425,868)	(270,230)
Proceeds from maturities and sales of short-term investments	333,967	309,653	336,261
Purchases of property and equipment	(10,580)	(4,513)	(5,733)
Proceeds from sale of property and equipment	0	0	161
Net cash provided by (used in) investing activities	22,258	(120,728)	60,459
Financing activities
Proceeds from sale of common stock and pre-funded warrants in underwritten offerings, net	0	353,780	140,888
Proceeds from issuance of common stock through ATM facilities, net	98,697	69,189	47,729
Proceeds from employee stock awards	6,762	6,680	7,350
Taxes paid related to net share settlement of restricted stock units	(1,244)	(1,521)	(6,695)
Principal payments on finance and capital lease obligations	(254)	(389)	(486)
Other financing activities, net	(17)	(165)	0
Net cash provided by financing activities	103,944	427,574	188,786
Increase (decrease) in cash, cash equivalents and restricted cash	(94,320)	126,087	13,619
Cash, cash equivalents and restricted cash at beginning of period	201,798	75,711	62,092
Cash, cash equivalents and restricted cash at end of period	107,478	201,798	75,711
Non-cash investing and financing activities
Property and equipment purchases included in accounts payable and other accrued liabilities	2,139	326	276
Accrued costs related to underwritten public offering	0	192	172
Accrued costs related to ATM facility	87	0	0
Proceeds from issuance of common stock through ATM facilities not yet received	0	0	1,185
Supplemental cash flow disclosure
Cash paid for interest	32	62	50
Cash paid for income taxes	$ 15	$ 10	$ 0