Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash flow from operating activities
Net loss	$ (71,005)	$ (68,112)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	13,934	6,544
Depreciation and amortization	2,371	2,722
Share of loss from equity method investee	613	1,202
Gain on deemed sale from equity method investee	(613)
Amortization of premiums on investments	(333)	(2,213)
Changes in operating assets and liabilities:
Receivable from collaboration partners	(2,447)	(5,276)
Amounts owed by a related party		83
Prepaid expenses and other current assets	(7,885)	(1,902)
Other long-term assets	(1,373)	21
Accounts payable	3,165	(94)
Accrued research and development expenses	23,665	2,989
Accrued liabilities	2,485	650
Other current liabilities	1,869	42
Deferred revenue	187,890	(250)
Deferred rent	741	(397)
Other long-term liabilities	7,973
Net cash (used in) provided by operating activities	161,050	(63,991)
Cash flow from investing activities
Purchases of short-term and long-term investments	(568,903)	(180,245)
Proceeds from maturities of short-term and long-term investments	135,300	253,290
Sales of short-term investments	1,003
Purchases of property and equipment	(1,536)	(1,729)
Net cash (used in) provided by investing activities	(434,136)	71,316
Cash flow from financing activities
Proceeds from issuance of common stock and rights to purchase additional shares, net of issuance costs	433,776
Proceeds from issuance of common stock pursuant to equity award plans	3,271	697
Repurchase of unvested shares of stock	(49)	(94)
Net cash provided by financing activities	436,998	603
Net increase in cash and cash equivalents	163,912	7,928
Cash, cash equivalents and restricted cash at beginning of period	58,140	71,267
Cash, cash equivalents and restricted cash at end of period	222,052	79,195
Non-cash investing and financing activities:
Unpaid portion of financing costs included in accrued liabilities	61
Unpaid portion of property and equipment purchases included in accounts payable and accrued liabilities	245	33
Vesting of early exercised stock options and restricted stock	$ 643	$ 784