Net Loss Per Share (Details 2) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Anti-dilutive shares excluded from the calculation of diluted net loss per share attributable to common stockholders
Shares excluded from the calculation of diluted net loss per share attributable to common stockholders	30,344	27,206	24,265
Options
Anti-dilutive shares excluded from the calculation of diluted net loss per share attributable to common stockholders
Shares excluded from the calculation of diluted net loss per share attributable to common stockholders	25,590	18,363	15,419
Common Stock Purchase Warrants
Anti-dilutive shares excluded from the calculation of diluted net loss per share attributable to common stockholders
Shares excluded from the calculation of diluted net loss per share attributable to common stockholders	3,926	5,931	5,154
Series A preferred stock
Anti-dilutive shares excluded from the calculation of diluted net loss per share attributable to common stockholders
Shares excluded from the calculation of diluted net loss per share attributable to common stockholders		2,857
Restricted stock awards
Anti-dilutive shares excluded from the calculation of diluted net loss per share attributable to common stockholders
Shares excluded from the calculation of diluted net loss per share attributable to common stockholders	828	55	122
Contingently redeemable common stock
Anti-dilutive shares excluded from the calculation of diluted net loss per share attributable to common stockholders
Shares excluded from the calculation of diluted net loss per share attributable to common stockholders			126
Subordinated secured convertible promissory notes
Anti-dilutive shares excluded from the calculation of diluted net loss per share attributable to common stockholders
Shares excluded from the calculation of diluted net loss per share attributable to common stockholders			3,444