CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash flows from operating activities:
Net loss	$ (70,671)	$ (50,122)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	3,899	2,908
Noncash lease expense	450
Stock-based compensation expense	6,336	3,213
Amortization of premium on short-term investments	(193)	(1,200)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	1,025	3,975
Accounts payable	4,920	576
Accrued expenses and other liabilities	1,888	302
Deferred revenue	(397)	(233)
Deferred rent		1,040
Operating lease liabilities	(1,090)
Net cash used in operating activities	(53,833)	(39,541)
Cash flows from investing activities:
Purchases of short-term investments	(19,992)	(189,475)
Maturities of short-term investments	189,620	187,225
Purchases of property and equipment	(2,621)	(16,385)
Net cash provided by investing activities	167,007	(18,635)
Cash flows from financing activities:
Proceeds from issuance of common stock in follow-on public offering, net of discounts and issuance costs		134,552
Proceeds from issuance of common stock pursuant to employee stock purchase plan	537	396
Proceeds from issuance of common stock from option exercises	151	76
Net cash provided by financing activities	688	135,024
Net change in cash, cash equivalents and restricted cash	113,862	76,848
Cash, cash equivalents and restricted cash, beginning of period	55,048	39,992
Cash, cash equivalents and restricted cash, end of period	168,910	116,840
Supplemental disclosures of noncash investing and financing activities:
Reclassification of liability for common stock vested	16	36
Property and equipment additions included in accounts payable	30	1,126
Property and equipment additions included in accrued expenses and other liabilities	7
Unrealized (loss) gain on available for sale securities, net	$ (172)	263
Deferred offering costs included in accounts payable and accrued expenses		$ 27