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Stock Incentive Plans - Stock Option Plan Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Stock options          
Outstanding at beginning of period (in shares)     1,723,554    
Granted (in shares)     1,043,045    
Exercised (in shares)     (51,732)    
Cancelled or forfeited (in shares)     (228,458)    
Outstanding at end of period (in shares) 2,486,409   2,486,409   1,723,554
Exercisable at end of period (in shares) 978,785   978,785    
Vested or expected to vest at end of period (in shares) 2,378,562   2,378,562    
Weighted-Average Exercise Price          
Outstanding at beginning of period (in dollars per share)     $ 6.66    
Granted (in dollars per share)     4.47    
Exercised (in dollars per share)     1.98    
Cancelled or forfeited (in dollars per share)     6.59    
Outstanding at end of period (in dollars per share) $ 5.84   5.84   $ 6.66
Exercisable at end of period (in dollars per share) 4.93   4.93    
Vested or expected to vest at end of period (in dollars per share) $ 5.83   $ 5.83    
Weighted-Average Remaining Contractual Term          
Outstanding at beginning of period     8 years 1 month 21 days   7 years 11 months 1 day
Outstanding at end of period     8 years 1 month 21 days   7 years 11 months 1 day
Exercisable at end of period     6 years 6 months 11 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 $ 4,037   4,037   $ 4,267
Exercisable at the end of the period $ 2,327   2,327    
Unrecognized stock-based compensation expense          
Fair value of options vested     $ 3,902    
Weighted average grant date fair value of options granted (in dollars per share) $ 2.84 $ 9.13 $ 2.88 $ 7.77  
Options to purchase Common stock          
Unrecognized stock-based compensation expense          
Intrinsic value of options exercised $ 30,000 $ 0 $ 200 $ 200  
Fair value of options vested 800 $ 300 1,900 $ 1,100  
Unrecognized compensation expense related to unvested stock option awards $ 5,400   $ 5,400    
Weighted-average amortization period over which cost is expected to be recognized     2 years 8 months 12 days