CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (57,215)	$ (29,991)
Adjustments to reconcile net loss to net cash provided by (used in) operating activities:
Depreciation	1,289	684
Stock-based compensation expense	2,623	277
Accretion on short-term investments	(1,589)	(85)
Loss on disposal of property and equipment	127
Change in fair value associated with convertible preferred stock tranche liability		876
Changes in operating assets and liabilities:
Prepaid expense and other current assets	(5,003)	(1,463)
Accounts payable	4,383	1,720
Accrued expenses and other liabilities	3,042	864
Deferred revenue	(252)	33,411
Deferred rent	10,033	186
Net cash (used in) provided by operating activities	(42,562)	6,479
Cash flows from investing activities:
Purchases of short-term investments	(245,328)	(78,071)
Maturities of short-term investments	148,480
Purchases of property and equipment	(24,939)	(1,958)
Changes in restricted cash		(1,497)
Net cash used in investing activities	(121,787)	(81,526)
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 from option exercises	87
Proceeds from issuance of restricted common stock	64
Repurchase of unvested common stock		(17)
Net cash provided by financing activities	150,994	115,230
Net change in cash and cash equivalents	(13,355)	40,183
Cash and cash equivalents, beginning of period	51,575	11,392
Cash and cash equivalents, end of period	38,220	51,575
Supplemental disclosures of noncash investing and financing activities:
Reclassification of liability for common stock vested	75	225
Property and equipment additions included in accounts payable	9,127	167
Deferred offering costs included in accrued expenses		1,000
Reclassification of tranche liability upon issuance of convertible preferred stock		5,123
Series A Convertible Preferred Stock
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs		20,479
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
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs		$ 94,768
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