Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flow from operating activities
Net loss	$ (122,858)	$ (84,710)	$ (49,594)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	21,825	8,981	3,874
Depreciation and amortization	3,149	3,578	3,664
Amortization of right-of-use assets	1,042
Share of loss (gain on deemed sale) from equity method investee, net		1,202	(343)
Amortization of premiums on investments	(46)	(2,638)	(1,752)
Other non-operating income			(177)
Changes in operating assets and liabilities:
Receivable from collaboration partners (($943), $0 and $0 from a related party)	(917)	(132)
Amounts owed by PACT Pharma		83	(58)
Prepaid expenses and other current assets	(838)	(1,982)	(1,180)
Other long-term assets	(5,374)	(588)	(80)
Accounts payable	9,272	1,726	(69)
Accrued research and development expenses	13,735	1,756	1,623
Other accrued liabilities	4,593	1,743	1,874
Other current liabilities		57	43
Deferred revenue ($185,379, $0 and $0 to a related party)	178,379	(2,000)	(353)
Operating lease liabilities	(993)
Deferred rent		(538)	(468)
Other long-term liabilities ($9,703, $0 and $0 to a related party)	10,201
Net cash provided by (used in) operating activities	111,170	(73,462)	(42,996)
Cash flow from investing activities
Purchases of short-term and long-term investments	(739,658)	(247,755)	(261,552)
Proceeds from maturities of short-term and long-term investments	307,343	308,892	151,855
Sales of short-term investments	1,003
Purchases of property and equipment	(3,055)	(1,925)	(3,743)
Net cash provided by (used in) investing activities	(434,367)	59,212	(113,440)
Cash flow from financing activities
Proceeds from initial public offering, net of issuance costs			125,111
Proceeds from issuance of common stock and rights to purchase additional shares ($164,207, $0 and $0 from a related party)	433,776
Proceeds from issuance of common stock pursuant to equity award plans	4,953	1,217	4,098
Repurchase of unvested shares of stock	(54)	(94)
Payment of preferred stock issuance costs			(135)
Net cash provided by financing activities	438,675	1,123	129,074
Net increase (decrease) in cash, cash equivalents and restricted cash	115,478	(13,127)	(27,362)
Cash, cash equivalents and restricted cash at beginning of period	58,140	71,267	98,629
Cash, cash equivalents and restricted cash at end of period	173,618	58,140	71,267
Supplemental disclosures:
Cash paid for amounts included in measurement of lease liabilities	2,105
Non-cash investing and financing activities:
Right-of-use assets obtained in exchange for new operating lease liabilities	8,019
Unpaid portion of financing costs included in accounts payable	61
Unpaid portion of property and equipment purchases included in accounts payable and accrued liabilities	1,583	12	136
Vesting of early exercised stock options and restricted stock	$ 848	$ 1,029	$ 1,276