CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash flows from operating activities:
Net loss	$ (128,694)	$ (103,916)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	7,965	6,318
Noncash lease expense	939
Stock-based compensation expense	13,248	7,635
Accretion of short-term investments	(198)	(1,871)
Loss on disposal of property and equipment	888	13
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	2,056	2,759
Accounts payable	2,459	(4,452)
Accrued expenses and other liabilities	2,432	2,315
Deferred revenue	6,824	(569)
Deferred rent		410
Operating lease liabilities	(2,251)
Net cash used in operating activities	(94,332)	(91,358)
Cash flows from investing activities:
Purchases of short-term investments	(19,991)	(286,391)
Maturities of short-term investments	228,620	256,425
Purchases of property and equipment	(3,733)	(21,833)
Net cash provided by (used in) investing activities	204,896	(51,799)
Cash flows from financing activities:
Proceeds from issuance of common stock in private placement	51,979
Proceeds from issuance of common stock in follow-on public offering, net of discounts and issuance costs		134,524
Proceeds from issuance of common stock from option exercises	176	351
Proceeds from issuance of common stock pursuant to employee stock purchase plan	938	829
Net cash provided by financing activities	53,093	158,213
Net change in cash, cash equivalents and restricted cash	163,657	15,056
Cash, cash equivalents and restricted cash, beginning of period	55,048	39,992
Cash, cash equivalents and restricted cash, end of period	218,705	55,048
Supplemental disclosures of noncash investing and financing activities:
Reclassification of liability for common stock vested	24	60
Property and equipment additions included in accounts payable	110	455
Property and equipment additions included in accrued expenses and other liabilities		249
Unrealized (loss) gain on available for sale securities, net	$ (183)	260
Offering costs included in accrued expenses and other liabilities		100
ATM
Cash flows from financing activities:
Proceeds from issuance of common stock in follow-on public offering, net of discounts and issuance costs		$ 22,509