Net Loss Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								6 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Jun. 30, 2014	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Basic and diluted net loss per common share calculation:
Net loss	$ (31,652)	$ 8,552	$ (72,990)						$ (93,672)	$ (85,120)	$ (116,772)	$ (51,145)	$ (14,562)
Accretion of preferred stock dividends												(5,891)	(7,063)
Net income (loss) attributable to common stockholders	$ (31,652)	$ 8,552	$ (72,990)	$ (20,682)	$ (20,388)	$ (18,424)	$ (11,863)	$ (6,361)			$ (116,772)	$ (57,036)	$ (21,625)
Weighted average common shares outstanding											33,258	9,003	1,452
Net income (loss) per share of common stock - basic and diluted (in dollars per share)					$ (0.65)	$ (10.26)	$ (8.07)	$ (4.33)			$ (3.51)	$ (6.34)	$ (14.89)
Anti-dilutive securities
Total (in shares)											3,731	2,796	18,101
Redeemable convertible preferred stock
Anti-dilutive securities
Total (in shares)													16,663
Options outstanding
Anti-dilutive securities
Total (in shares)											3,680	2,708	1,344
Warrants
Anti-dilutive securities
Total (in shares)											14	88	94
Restricted stock units
Anti-dilutive securities
Total (in shares)											37