Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Activities:
Net loss	$ (139,700)	$ (77,270)	$ (74,518)
Adjustments to reconcile net loss to net cash used in operating activities:
Accretion of discount on convertible notes	2,758	3,602	3,059
Depreciation and amortization expense	1,145	426	330
Write down of property and equipment	419	0	0
Stock-based compensation	18,567	13,371	13,601
Expense in Connection with Warrant Issue	26,562	0	0
Loss on disposal of property and equipment	0	0	317
Accretion of discount (amortization of premium) on investments, net	580	(1,066)	(801)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(1,517)	(917)	(1,505)
Accounts payable and accrued expenses	11,015	(2,722)	5,890
Operating lease liabilities	(139)	(87)	(161)
Finance lease liability	5,557	0	0
Other long term liabilities	113	0	0
Net cash used in operating activities	(74,640)	(64,663)	(53,788)
Investing activities:
Cash acquired in connection with the Reverse Merger	0	0	76,348
Purchases of investments	(209,343)	(184,298)	(141,087)
Proceeds from maturities of investments	141,811	168,556	61,276
Payments made to acquire right of use asset	(8,452)	0	0
Proceeds from sale of property and equipment	0	0	20
Purchases of property and equipment	(20,607)	(23,269)	(1,453)
Payment of security deposit	0	0	(376)
Net cash used in investing activities	(96,591)	(39,011)	(5,272)
Financing activities:
Issuance of common stock, net of issuance costs	280,763	177,760	154,022
Issuance of common stock, pursuant to exercise of stock options	2,558	31	148
Common stock repurchase	(72)	0	0
Treasury stock repurchase	0	0	(668)
Proceeds from sale of treasury stock	591	344	0
Payment of withholding tax on option exercise	(614)	(344)	0
Convertible notes refinancing costs to the lender	(237)	0	0
Net cash provided by financing activities	282,989	177,791	153,502
Net change in cash, cash equivalents and restricted cash	111,758	74,117	94,442
Cash, cash equivalents and restricted cash at beginning of period	186,908	112,791	18,349
Cash, cash equivalents and restricted cash at end of period	298,666	186,908	112,791
Supplemental disclosure of non-cash financing and investing activities:
Accrued purchases of property and equipment	1,456	4,424	0
Accrued purchases of internal use software	300	226	0
Treasury stock purchases paid in prior year	0	726	0
Retirement of treasury stock	0	1,393	0
Witholding tax payable on shares witheld in treasury stock	0	53	0
Conversion of convertible preferred stock into common stock	0	0	41,466
Conversion of convertible notes into common stock	7,629	0	0
Unrealized (loss) gain on investments	(62)	147	(127)
Finance lease right of use asset and lease liability	20,179	0	0
Reclassification of construction in process to finance right of use asset	26,465	0	0
Supplemental cash flow information:
Cash paid for interest	2,960	2,990	4,485
Cash paid for income taxes	$ 0	$ 26	$ 2