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Stock Incentive Plans - Additional Information (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Stock incentive plans      
Weighted average fair value of options to purchase common stock granted (in dollars per share) $ 3.35 $ 5.12 $ 9.08
Intrinsic value of stock options exercised (in dollars) $ 0.9 $ 1.8 $ 3.2
Weighted average grant date fair value (in usd per share) $ 7.47    
Common Stock      
Stock incentive plans      
Share Price (in usd per share) $ 7.74 $ 7.20  
Employee stock options      
Stock incentive plans      
Total estimated grant date fair value of options to purchase common stock vested (in dollars) $ 5.8 $ 5.3 $ 1.6