Condensed Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Cash flows from operating activities:
Net loss	$ (39,188)	$ (24,842)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	336	225
Stock-based compensation expense	1,984	653
Changes in operating assets and liabilities:
Prepaid expenses and other assets	356	(613)
Accounts receivable	344	(262)
Accounts payable	(263)	839
Accrued expenses and other liabilities	3,966	(336)
Net cash used in operating activities	(32,465)	(24,336)
Cash flows from investing activities:
Purchases of property and equipment	(391)	(1,338)
Net cash used in investing activities	(391)	(1,338)
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 stock options exercised	2
Proceeds from initial public offering, net of underwriters discounts	109,517
Payment of deferred offering costs	(2,971)
Net cash provided by financing activities	106,316	39,979
Net increase in cash, cash equivalents, and restricted cash	73,460	14,305
Cash, cash equivalents and restricted cash at beginning of period	92,609	16,953
Cash, cash equivalents and restricted cash at end of period	166,069	31,258
Supplemental disclosure of non-cash investing and financing activities:
Property and equipment purchases not yet paid		$ 34
Deferred initial public offering costs not yet paid	$ 41