Net Loss Per Share - Basic and Diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Numerator:
Net loss	$ (35,474)	$ (68,882)	$ (68,882)	$ (78,234)	$ (130,437)
Denominator:
Weighted-average number of common shares used in loss per share - basic and diluted (in shares)	46,109,193	45,430,678		45,891,557	45,185,588
Loss per share - basic and diluted (in dollars per share)	$ (0.77)	$ (1.52)		$ (1.70)	$ (2.89)