Condensed Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Cash flows from operating activities:
Net loss	$ (25,007)	$ (16,696)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	216	134
Stock-based compensation expense	1,264	412
Changes in operating assets and liabilities:
Prepaid expenses and other assets	1,154	(88)
Accounts receivable	(90)	(486)
Accounts payable	109	(93)
Accrued expenses and other current liabilities	2,144	1,021
Net cash used in operating activities	(20,210)	(15,796)
Cash flows from investing activities:
Purchases of property and equipment	(322)	(958)
Net cash used in investing activities	(322)	(958)
Cash flows from financing activities:
Proceeds from issuance of Series A-2 convertible preferred stock, net issuance of costs		39,979
Payment of deferred issuance costs associated with Series B convertible preferred stock financing	(232)
Proceeds from initial public offering, net of underwriters discounts	109,517
Payment of deferred offering costs	(1,799)
Net cash provided by financing activities	107,486	39,979
Net increase in cash, cash equivalents, and restricted cash	86,954	23,225
Cash, cash equivalents and restricted cash at beginning of period	92,609	16,953
Cash, cash equivalents and restricted cash at end of period	179,563	40,178
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases not yet paid		$ 177
Deferred initial public offering costs not yet paid	$ 1,103