CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net loss	$ (79,749)	$ (36,791)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	4,599	726
Stock-based compensation expense	5,199	1,668
Amortization of premium on short-term investments	(1,601)	(389)
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	2,541	(6,378)
Accounts payable	1,511	(571)
Accrued expenses and other liabilities	2,121	1,831
Deferred revenue	(379)	(1,484)
Deferred rent	733	6,905
Net cash used in operating activities	(65,025)	(34,483)
Cash flows from investing activities:
Purchases of short-term investments	(257,389)	(232,460)
Maturities of short-term investments	243,725	109,480
Purchases of property and equipment	(18,108)	(9,009)
Net cash used in investing activities	(31,772)	(131,989)
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	134,524
Proceeds from issuance of restricted common stock		65
Proceeds from issuance of common stock from option exercises	285	45
Proceeds from issuance of common stock pursuant to employee stock purchase plan	829
Net cash provided by financing activities	135,638	150,953
Net change in cash, cash equivalents and restricted cash	38,841	(15,519)
Cash, cash equivalents and restricted cash, beginning of period	39,992	53,347
Cash, cash equivalents and restricted cash, end of period	78,833	37,828
Supplemental disclosures of noncash investing and financing activities:
Reclassification of liability for common stock vested	47	57
Property and equipment additions included in accounts payable	682	9,471
Property and equipment additions included in accrued expenses and other liabilities	705
Unrealized gain on available for sale securities, net	$ 331	52
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