Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities:
Net loss	$ (241,846)	$ (202,244)	$ (83,948)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation expense	26,873	20,730	22,759
Depreciation and amortization expense	405	471	112
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(325)	138	332
Accounts payable	20,363	(161)	(1,309)
Accrued expense	9,826	21,585	16,783
Accrued interest, net of interest received on maturity of investments	787	1,920	3,647
Net cash used in operating activities	(183,917)	(157,561)	(41,624)
Cash flows from investing activities:
Purchases of marketable securities	(394,120)	(329,342)	(619,303)
Sales and maturities of marketable securities	389,274	489,456	650,182
Purchases of property and equipment, net of disposals	(209)	(334)	(172)
Net cash provided by (used in) investing activities	(5,055)	159,780	30,707
Cash flows from financing activities:
Proceeds from issuances of stock, excluding related parties, net of transaction costs	170,207	4,421
Proceeds from the sale of related party stock and exercise of common stock options, net of transaction costs	1,030	667	235
Net cash provided by financing activities	171,237	5,088	235
Net increase (decrease) in cash and cash equivalents	(17,735)	7,307	(10,682)
Cash and cash equivalents at beginning of period	54,004	46,697	57,379
Cash and cash equivalents at end of period	36,269	54,004	46,697
Supplemental disclosure of cash flow information:
Obtaining a right-of-use asset in exchange for a lease liability	$ 376	$ 451	900
Purchases of property and equipment in accounts payable at period end			$ 897