Net Loss Per Share - Basic and Diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net loss	$ (57,843)	$ (46,186)	$ (183,220)	$ (130,084)
Denominator:
Weighted-average number of common shares used in loss per share - basic and diluted (in shares)	43,999,451	43,092,921	43,535,874	43,049,734
Loss per share - basic and diluted (in dollars per share)	$ (1.31)	$ (1.07)	$ (4.21)	$ (3.02)