Net Loss Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Numerator:
Net loss	$ (33,240)	$ (28,264)	$ (22,965)	$ (17,057)	$ (17,962)	$ (17,420)	$ (12,609)	$ (14,488)	$ (101,526)	$ (62,479)	$ (60,690)
Accretion of Preferred Stock										(9,000)	(17,471)
Loss attributable to common stockholders - basic	$ (33,240)	$ (28,264)	$ (22,965)	$ (17,057)	$ (17,962)	$ (17,420)	$ (16,640)	$ (19,457)	(101,526)	(71,479)	(78,161)
Earnings (loss) attributable to common stockholders - diluted									$ (101,526)	$ (71,479)	$ (78,161)
Denominator:
Weighted-average number of common shares used in (loss) earnings per share - diluted									39,643,099	17,699,487	383,310
Basic and diluted (in dollars per share)									$ (2.56)	$ (4.04)	$ (203.91)