Statements of Stockholders' (Deficit) Equity - USD ($) shares in Thousands, $ in Thousands	Common stock	Additional paid-in capital	Accumulated deficit	Total
Stockholders' (deficit) equity beginning balance at Dec. 31, 2016	$ 50	$ 11,588	$ (20,189)	$ (8,551)
Stockholders' (deficit) equity shares beginning balance at Dec. 31, 2016	5,049
Stockholders' (Deficit) Equity Rollforward
Issuance of common stock upon vesting of restricted stock awards	$ 3	(3)
Issuance of common stock upon vesting of restricted stock awards (in shares)	293
Stock-based compensation		901		901
Net loss			(32,068)	$ (32,068)
Stockholders' (deficit) equity shares ending balance at Dec. 31, 2017	5,342			5,342
Stockholders' (deficit) equity ending balance at Dec. 31, 2017	$ 53	12,486	(52,257)	$ (39,718)
Stockholders' (Deficit) Equity Rollforward
Issuance of common stock upon vesting of restricted stock awards	$ 2	(2)
Issuance of common stock upon vesting of restricted stock awards (in shares)	147
Stock-based compensation		1,264		1,264
Conversion of preferred stock upon IPO	$ 203	132,183		132,386
Conversion of preferred stock upon IPO (in shares)	20,306
Issuance of common stock upon IPO, net of underwriters' discount and other offering costs of $2,902	$ 74	106,542		106,616
Issuance of common stock upon IPO, net of underwriters' discount and other offering costs of $2,902 (in shares)	7,360
Net loss			(25,007)	$ (25,007)
Stockholders' (deficit) equity shares ending balance at Jun. 30, 2018	33,155			33,155
Stockholders' (deficit) equity ending balance at Jun. 30, 2018	$ 332	$ 252,473	$ (77,264)	$ 175,541