Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Operating Activities:
Net loss	$ (74,704)	$ (49,704)
Adjustments to reconcile net loss to net cash used in operating activities:
Accretion of discount on convertible notes	705	1,416
Depreciation and amortization expense	1,380	242
Write down of property and equipment	0	62
Stock-based compensation	15,211	8,450
Accretion of discount on investments, net	1,345	63
Unrealized loss on marketable securities	1	0
Changes in operating assets and liabilities:
Prepaid expenses and other assets	1,030	(2,522)
Accounts payable and accrued expenses	(8,982)	(3,188)
Operating lease liabilities	50	(94)
Finance lease liability	1,184	997
Other long term liabilities	(29)	0
Net cash used in operating activities	(62,809)	(44,278)
Investing activities:
Purchases of investments	(158,571)	(56,436)
Proceeds from maturities of investments	180,584	88,714
Payments made to acquire right of use asset	(61)	(2,731)
Purchases of property and equipment	(1,449)	(7,115)
Purchases of internal use software	(325)	(368)
Net cash provided by investing activities	20,178	22,064
Financing activities:
Issuance of common stock, pursuant to exercise of stock options	9,907	3
Issuance of common stock, net of issuance costs	0	287
Common stock repurchase	0	(72)
Proceeds from sale of treasury stock	0	591
Payment of withholding tax on option exercise	0	(538)
Convertible notes refinancing costs to the lender	0	(237)
Net cash provided by financing activities	9,907	34
Net change in cash, cash equivalents and restricted cash	(32,724)	(22,180)
Cash, cash equivalents and restricted cash at beginning of period	298,666	186,908
Cash, cash equivalents and restricted cash at end of period	265,942	164,728
Supplemental disclosure of non-cash financing and investing activities:
Accrued purchases of property and equipment	1,272	3,958
Accrued purchases of internal use software	122	122
Unrealized (loss) gain on investments	(39)	209
Conversion of convertible notes into common stock	35,544	0
Finance lease right of use asset and lease liability	0	20,179
Reclassification of construction in process (from) to finance right of use asset	(5)	26,465
Supplemental cash flow information:
Cash paid for interest	$ 1,347	$ 1,495