Net Loss Per Share - Basic and Diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Numerator:
Net loss	$ (42,760)	$ (61,555)
Denominator:
Weighted-average number of common shares used in loss per share - basic and diluted (in shares)	45,671,502	44,937,776
Loss per share - basic and diluted (in dollars per share)	$ (0.94)	$ (1.37)