CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS (UNAUDITED) - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities:
Net loss	$ (38,213,561)	$ (18,085,115)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	725,825	483,389
Stock-based compensation expense	1,667,790	116,142
Accretion on short-term investments	(389,431)
Change in fair value associated with convertible preferred stock tranche liability		876,000
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(6,377,573)	(267,257)
Accounts payable	(571,193)	531,357
Accrued expenses and other liabilities	1,831,239	653,947
Deferred revenue	(61,305)
Deferred rent	6,905,160	122,549
Net cash used in operating activities	(34,483,049)	(15,568,988)
Cash flows from investing activities:
Purchases of short-term investments	(232,460,163)
Maturities of short-term investments	109,480,000
Purchases of property and equipment	(9,008,873)	(1,621,265)
Changes in restricted cash	(17,375)
Net cash used in investing activities	(132,006,411)	(1,621,265)
Cash flows from financing activities:
Proceeds from issuance of common stock in initial public offering, net of discounts and issuance costs	150,843,215
Proceeds from issuance of common stock from option exercises	45,206
Proceeds from issuance of restricted common stock	64,498
Repurchase of unvested common stock		(17,470)
Net cash provided by financing activities	150,952,919	103,567,044
Net change in cash and cash equivalents	(15,536,541)	86,376,791
Cash and cash equivalents, beginning of period	51,574,932	11,392,207
Cash and cash equivalents, end of period	36,038,391	97,768,998
Supplemental disclosures of noncash investing and financing activities:
Reclassification of liability for common stock vested	56,986	20,605
Property and equipment additions included in accounts payable	9,470,859	137,095
Reclassification of tranche liability upon issuance of convertible preferred stock		5,123,000
Series A Convertible Preferred Stock
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs		20,479,488
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	42,994,550
Series B Convertible Preferred Stock
Cash flows from financing activities:
Proceeds from issuance of convertible preferred stock, net of issuance costs		$ 83,105,026
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	$ 94,767,610