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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net loss	$ (25,467)	$ (22,291)	$ (51,611)	$ (39,310)
Denominator:
Weighted average common shares outstanding - basic and diluted	62,651,863	52,824,483	62,599,371	50,206,470
Net loss per share - basic and diluted	$ (0.41)	$ (0.42)	$ (0.82)	$ (0.78)