CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Cash flows from operating activities:
Net loss	$ (50,122)	$ (22,643)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	2,908	458
Stock-based compensation expense	3,213	949
Accretion of discount (amortization of premium) on short-term investments	(1,200)	111
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	3,975	(4,130)
Accounts payable	576	2,567
Accrued expenses and other liabilities	302	355
Deferred revenue	(233)	(938)
Deferred rent	1,040	2,480
Net cash used in operating activities	(39,541)	(20,791)
Cash flows from investing activities:
Purchases of short-term investments	(189,475)	(143,770)
Maturities of short-term investments	187,225	61,220
Purchases of property and equipment	(16,385)	(4,184)
Net cash used in investing activities	(18,635)	(86,734)
Cash flows from financing activities:
Proceeds from issuance of common stock in initial public offering, net of discounts and issuance costs		150,843
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 from option exercises	76	85
Proceeds from issuance of common stock pursuant to employee stock purchase plan	396
Net cash provided by financing activities	135,024	150,928
Net change in cash, cash equivalents and restricted cash	76,848	43,403
Cash, cash equivalents and restricted cash, beginning of period	39,992	53,347
Cash, cash equivalents and restricted cash, end of period	116,840	96,750
Supplemental disclosures of noncash investing and financing activities:
Reclassification of liability for common stock vested	36	6
Property and equipment additions included in accounts payable	1,126	230
Deferred offering costs included in accounts payable and accrued expenses	$ 27
Series A Convertible Preferred Stock | Initial Public Offering
Supplemental disclosures of noncash investing and financing activities:
Conversion of convertible preferred stock into common stock upon initial public offering		42,994
Series B Convertible Preferred Stock | Initial Public Offering
Supplemental disclosures of noncash investing and financing activities:
Conversion of convertible preferred stock into common stock upon initial public offering		$ 94,768