Net Loss Per Common Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Basic and diluted net loss per common share calculation:
Net loss	$ (6,445)	$ (8,566)	$ (4,691)	$ (5,169)	$ (7,976)	$ (4,483)	$ (9,304)	$ (9,021)	$ (24,871)	$ (30,784)	$ (71,865)
Net loss attributable to common stockholders									$ (24,871)	$ (30,784)	$ (71,865)
Weighted average common shares outstanding (in shares)	92,777,480	92,569,993	92,414,644	88,897,292	82,323,393	77,445,675	69,664,994	64,562,236	91,677,963	73,558,548	59,436,649
Net loss per share of common stock, basic and diluted (in dollars per share)	$ (0.07)	$ (0.09)	$ (0.05)	$ (0.06)	$ (0.10)	$ (0.06)	$ (0.13)	$ (0.14)	$ (0.27)	$ (0.42)	$ (1.21)
Outstanding securities excluded from computation of diluted weighted shares outstanding (in shares)									11,136,980	9,643,298	8,747,314
Employee Stock Option
Basic and diluted net loss per common share calculation:
Outstanding securities excluded from computation of diluted weighted shares outstanding (in shares)									7,568,304	8,265,207	8,624,223
Restricted Stock Units
Basic and diluted net loss per common share calculation:
Outstanding securities excluded from computation of diluted weighted shares outstanding (in shares)									2,945,585	1,255,000
Warrants
Basic and diluted net loss per common share calculation:
Outstanding securities excluded from computation of diluted weighted shares outstanding (in shares)									623,091	123,091	123,091