Net Loss Per Share (Details 2)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Convertible preferred stock      
Net Loss Per Share      
Potentially dilutive securities, prior to the use of the treasury stock method, excluded from the computation of diluted weighted-average shares outstanding, as they would be anti-dilutive 3,857,664 6,617,686 3,412,898
Options to purchase common stock      
Net Loss Per Share      
Potentially dilutive securities, prior to the use of the treasury stock method, excluded from the computation of diluted weighted-average shares outstanding, as they would be anti-dilutive 2,466,492 1,743,890 1,706,539
Warrants      
Net Loss Per Share      
Potentially dilutive securities, prior to the use of the treasury stock method, excluded from the computation of diluted weighted-average shares outstanding, as they would be anti-dilutive 1,271,520 545,797 15,000