Net Loss Per Share - Computation of Basic and Diluted Loss Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net loss	$ (23,539)	$ (24,126)	$ (75,150)	$ (63,436)
Denominator:
Weighted average common shares outstanding - basic and diluted	62,828,513	62,071,397	62,676,591	54,204,907
Net loss per share - basic and diluted	$ (0.37)	$ (0.39)	$ (1.20)	$ (1.17)