Net Loss Per Share - Basic and Diluted (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	$ (49,802)	$ (57,843)	$ (180,239)	$ (183,220)
Denominator:
Weighted-average number of common shares used in loss per share - basic and diluted (in shares)	45,498,909	43,999,451	45,291,176	43,535,874
Loss per share - basic and diluted (in dollars per share)	$ (1.09)	$ (1.31)	$ (3.98)	$ (4.21)