CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities:
Net loss	$ (35,331)	$ (23,852)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	1,907	1,266
Amortization of right-of-use assets	221
Stock-based compensation expense	2,883	1,243
Amortization of premium on short-term investments	(175)	(661)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	1,087	4,413
Accounts payable	4,841	(85)
Accrued expenses and other liabilities	(535)	(1,404)
Deferred revenue	(203)	(92)
Deferred rent		1,301
Operating lease liabilities	(541)
Net cash used in operating activities	(25,846)	(17,871)
Cash flows from investing activities:
Purchases of short-term investments		(84,467)
Maturities of short-term investments	117,030	114,600
Purchases of property and equipment	(2,033)	(10,466)
Net cash provided by investing activities	114,997	19,667
Cash flows from financing activities:
Proceeds from issuance of common stock pursuant to employee stock purchase plan	537	396
Proceeds from issuance of common stock from option exercises	135	21
Payment of deferred offering costs	225	92
Net cash provided by financing activities	447	325
Net change in cash, cash equivalents and restricted cash	89,598	2,121
Cash, cash equivalents and restricted cash, beginning of period	55,048	39,992
Cash, cash equivalents and restricted cash, end of period	144,646	42,113
Supplemental disclosures of noncash investing and financing activities:
Reclassification of liability for common stock vested	12	18
Property and equipment additions included in accounts payable	405	5,274
Property and equipment additions included in accrued expenses and other liabilities	23
Unrealized gain on available for sale securities, net	(202)	83
Deferred offering costs included in accounts payable and accrued expenses	$ 77	$ 174