Net Loss Per Share - Basic and Diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Numerator:
Net loss	$ (46,186)	$ (28,264)	$ (130,084)	$ (68,286)
Loss attributable to common stockholders - basic and diluted	$ (46,186)	$ (28,264)	$ (130,084)	$ (68,286)
Denominator:
Weighted-average number of common shares used in loss per share - basic and diluted (in shares)	43,092,921	41,331,612	43,049,734	38,525,827
Loss per share - basic and diluted (in dollars per share)	$ (1.07)	$ (0.68)	$ (3.02)	$ (1.77)