Net Loss per Share - Computation of Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Numerator
Net loss	$ (30,835)	$ (12,308)	$ (54,963)	$ (30,993)
Denominator
Weighted-average number of common shares used in net loss per share – basic	28,704,853	24,845,364	27,022,269	23,196,293
Weighted-average number of common shares used in net loss per share – diluted	28,704,853	24,845,364	27,022,269	23,196,293
Net loss per share – basic	$ (1.07)	$ (0.50)	$ (2.03)	$ (1.34)
Net loss per share – diluted	$ (1.07)	$ (0.50)	$ (2.03)	$ (1.34)