Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Net loss	$ (52,720)	$ (46,186)	$ (43,435)	$ (40,463)	$ (33,240)	$ (28,264)	$ (22,965)	$ (17,057)	$ (182,804)	$ (101,526)	$ (62,479)
Accretion of preferred stock									0	0	(9,000)
Loss attributable to common stockholders—basic	$ (52,720)	$ (46,186)	$ (43,435)	$ (40,463)	$ (33,240)	$ (28,264)	$ (22,965)	$ (17,057)	(182,804)	(101,526)	(71,479)
Effect of dilutive convertible preferred stock									0	0	0
Loss attributable to common stockholders—diluted									$ (182,804)	$ (101,526)	$ (71,479)
Denominator:
Weighted-average number of common shares used in loss per share— basic and diluted	43,122,210	43,092,921	43,042,883	43,012,924	42,924,137	41,331,612	37,895,651	36,268,975	43,067,952	39,643,099	17,699,487
Loss per share—basic and diluted	$ (1.22)	$ (1.07)	$ (1.01)	$ (0.94)	$ (0.77)	$ (0.68)	$ (0.61)	$ (0.47)	$ (4.24)	$ (2.56)	$ (4.04)