Net Loss Per Share - Computation of Basic and Diluted Loss Per Share Attributable to Common Stockholders (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss	$ (27,862)	$ (24,126)	$ (22,291)	$ (17,019)	$ 10,114	$ (20,724)	$ (25,721)	$ (28,008)	$ (91,298)	$ (64,339)	$ (112,387)
Denominator:
Weighted average common shares outstanding - basic and diluted									56,219,919	47,435,331	40,118,522
Net loss per share - basic and diluted	$ (0.45)	$ (0.39)	$ (0.42)	$ (0.36)					$ (1.62)	$ (1.36)	$ (2.80)