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Stock Incentive Plans - Stock Option Plan Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Stock options      
Outstanding at beginning of period (in shares) 1,723,554    
Granted (in shares) 514,145    
Exercised (in shares) (28,119)    
Cancelled or forfeited (in shares) (12,550)    
Outstanding at end of period (in shares) 2,197,030   1,723,554
Exercisable at end of period (in shares) 870,918    
Vested or expected to vest at end of period (in shares) 2,013,343    
Weighted-Average Exercise Price      
Outstanding at beginning of period (in dollars per share) $ 6.66    
Granted (in dollars per share) 4.57    
Exercised (in dollars per share) 2.21    
Cancelled or forfeited (in dollars per share) 7.29    
Outstanding at end of period (in dollars per share) 6.22   $ 6.66
Exercisable at end of period (in dollars per share) 3.97    
Vested or expected to vest at end of period (in dollars per share) $ 6.09    
Weighted-Average Remaining Contractual Term      
Outstanding at beginning of period 8 years 2 months 12 days   7 years 11 months 1 day
Outstanding at end of period 8 years 2 months 12 days   7 years 11 months 1 day
Exercisable at end of period 6 years 8 months 16 days    
Vested or expected to vest at end of period 8 years 1 month 2 days    
Aggregate Intrinsic Value      
Outstanding at the beginning of the period $ 4,267    
Outstanding at the end of the period 2,070   $ 4,267
Exercisable at the end of the period 1,695    
Unrecognized stock-based compensation expense      
Fair value of options vested $ 2,025    
Weighted average grant date fair value of options granted (in dollars per share) $ 2.96 $ 6.93  
Options to purchase Common stock      
Unrecognized stock-based compensation expense      
Intrinsic value of options exercised $ 100 $ 200  
Fair value of options vested 500 $ 600  
Unrecognized compensation expense related to unvested stock option awards $ 5,600    
Weighted-average amortization period over which cost is expected to be recognized 2 years 9 months 18 days