Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Operating Activities:
Net loss	$ (40,179)	$ (24,664)
Adjustments to reconcile net loss to net cash used in operating activities:
Accretion of discount on convertible notes	589	844
Depreciation and amortization expense	674	118
Write down of property and equipment	0	63
Stock-based compensation	7,900	3,961
Accretion of discount (amortization of premium) on investments, net	620	(63)
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(842)	(271)
Accounts payable and accrued expenses	7,047	(1,932)
Operating lease liabilities	(15)	(452)
Finance lease liability	(63)	151
Other long term liabilities	(14)	0
Net cash used in operating activities	(24,283)	(22,245)
Investing activities:
Purchases of investments	(103,815)	(23,481)
Proceeds from maturities of investments	74,969	56,805
Payments made to acquire right of use asset	0	(530)
Purchases of property and equipment	(328)	(5,207)
Purchases of internal use software	0	(88)
Net cash (used in) provided by investing activities	(29,174)	27,499
Financing activities:
Issuance of common stock, pursuant to exercise of stock options	8,792	0
Common stock repurchase	0	(72)
Proceeds from sale of treasury stock	0	53
Convertible notes refinancing costs to the lender	0	(98)
Net cash provided by (used in) financing activities	8,792	(117)
Net change in cash, cash equivalents and restricted cash	(44,665)	5,137
Cash, cash equivalents and restricted cash at beginning of period	298,666	186,908
Cash, cash equivalents and restricted cash at end of period	254,001	192,045
Supplemental disclosure of non-cash financing and investing activities:
Accrued purchases of property and equipment	337	2,796
Accrued purchases of internal use software	311	174
Retirement of treasury stock	0	72
Witholding tax payable on shares witheld in treasury stock	0	429
Unrealized loss on investments	(33)	(95)
Finance lease right of use asset and lease liability	0	20,179
Reclassification of construction in process to finance right of use asset	325	26,465
Supplemental cash flow information:
Cash paid for interest	$ 1,347	$ 1,495