Net Loss Per Share - Basic and Diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Numerator:
Net loss	$ (30,044)	$ (49,802)	$ (108,278)	$ (180,239)
Denominator:
Weighted-average number of common shares used in loss per share - basic and diluted (in shares)	46,141,217	45,498,909	45,975,691	45,291,176
Loss per share - basic and diluted (in dollars per share)	$ (0.65)	$ (1.09)	$ (2.36)	$ (3.98)