Loss per Common Share - Computation of Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Loss per Common Share
Net loss	$ (6,109)	$ (4,712)	$ (9,700)	$ (16,502)	$ (13,060)	$ (18,652)	$ (20,521)	$ (48,214)
Weighted-average number of common shares used in net loss per share - basic and diluted	33,481,923			33,012,174			33,360,272	32,950,584
Loss per share - basic and diluted	$ (0.18)			$ (0.50)			$ (0.62)	$ (1.46)