Basic and Diluted Loss per Common Share - Computation of Net Loss per 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
Numerator:
Net loss	$ (10,797)	$ (14,650)	$ (10,051)	$ (15,216)	$ (13,903)	$ (15,028)	$ (10,967)	$ (9,553)	$ (50,714)	$ (49,451)	$ (31,007)
Accretion of dividends on convertible preferred stock									0	0	357
Net loss attributable to common stockholders									$ (50,714)	$ (49,451)	$ (31,364)
Denominator
Weighted average common shares outstanding, basic and diluted (in shares)									65,479,327	62,140,684	49,529,833
Net loss per share attributable to common stockholders, basic and diluted (in usd per share)									$ (0.77)	$ (0.80)	$ (0.63)