CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Cash flows from operating activities:
Net loss	$ (98,903)	$ (79,749)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	5,909	4,599
Noncash lease expense	686
Stock-based compensation expense	9,682	5,199
Amortization of premium on short-term investments	(197)	(1,601)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	938	2,541
Accounts payable	486	1,511
Accrued expenses and other liabilities	924	2,121
Deferred revenue	(590)	(379)
Deferred rent		733
Operating lease liabilities	(1,655)
Net cash used in operating activities	(82,720)	(65,025)
Cash flows from investing activities:
Purchases of short-term investments	(19,991)	(257,389)
Maturities of short-term investments	228,620	243,725
Purchases of property and equipment	(2,778)	(18,108)
Net cash provided by (used in) investing activities	205,851	(31,772)
Cash flows from financing activities:
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 pursuant to employee stock purchase plan	938	829
Proceeds from issuance of common stock from option exercises	160	285
Net cash provided by financing activities	1,098	135,638
Net change in cash, cash equivalents and restricted cash	124,229	38,841
Cash, cash equivalents and restricted cash, beginning of period	55,048	39,992
Cash, cash equivalents and restricted cash, end of period	179,277	78,833
Supplemental disclosures of noncash investing and financing activities:
Reclassification of liability for common stock vested	21	47
Property and equipment additions included in accounts payable	677	682
Property and equipment additions included in accrued expenses and other liabilities	126	705
Unrealized (loss) gain on available for sale securities, net	$ (183)	$ 331