Summary of Significant Accounting Policies - Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Summary of Significant Accounting Policies
Net loss	$ (17,686)	$ (10,016)	$ (48,557)	$ (23,178)
Cumulative dividends on convertible preferred shares		(1,413)		(3,785)
Loss attributable to common shares—basic and diluted	$ (17,686)	$ (11,429)	$ (48,557)	$ (26,963)
Weighted—average number of common shares—basic and diluted	34,256,519	6,404,254	29,859,314	8,918,389
Loss per common share—basic and diluted	$ (0.52)	$ (1.78)	$ (1.63)	$ (3.02)