CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash flows from operating activities:
Net loss	$ (23,852)	$ (10,217)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	1,266	221
Stock-based compensation expense	1,243	380
Accretion on short-term investments	(661)	(123)
Changes in operating assets and liabilities:
Prepaid expense and other current assets	4,413	(1,728)
Accounts payable	(85)	511
Accrued expenses and other liabilities	(1,404)	(764)
Deferred revenue	(92)	(463)
Deferred rent	1,301	1,139
Net cash used in operating activities	(17,871)	(11,044)
Cash flows from investing activities:
Purchases of short-term investments	(84,467)	(18,595)
Maturities of short-term investments	114,600	26,270
Purchases of property and equipment	(10,466)	(1,035)
Net cash provided by investing activities	19,667	6,640
Cash flows from financing activities:
Proceeds from issuance of common stock from option exercises	21	69
Proceeds from issuance of common stock pursuant to employee stock purchase plan	396
Payment of deferred offering costs	(92)	(1,027)
Net cash provided by (used in) financing activities	325	(958)
Net change in cash, cash equivalents and restricted cash	2,121	(5,362)
Cash, cash equivalents and restricted cash, beginning of period	39,992	53,347
Cash, cash equivalents and restricted cash, end of period	42,113	47,985
Supplemental disclosures of noncash investing and financing activities:
Reclassification of liability for common stock vested	18	16
Property and equipment additions included in accounts payable	5,274	27
Deferred offering costs included in accounts payable and accrued expenses	$ 174	$ 2,046