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Stock Based Compensation - Stock Option Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]    
Number of options outstanding, beginning balance (in shares) 3,582,241  
Number of options, granted (in shares) 179,922  
Number of options exercised (in shares) (58,162)  
Number of options, forfeited or cancelled (in shares) (44,210)  
Number of options outstanding, ending balance (in shares) 3,659,791 3,582,241
Number of options outstanding, options vested and exercisable (in shares) 3,209,890  
Number of options outstanding, options vested and expected to vest (in shares) 3,641,078  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]    
Weighted-average exercise price, ending balance (in usd per share) $ 6.54  
Weighted-average exercise price, granted (in usd per share) 13.79  
Weighted-average exercise price, exercised (in usd per share) 5.27  
Weighted-average exercise price, forfeited or cancelled (in usd per share) 9.46  
Weighted-average exercise price, ending balance (in usd per share) 6.88 $ 6.54
Weighted-average exercise price, options vested and exercisable (in usd per share) 6.28  
Weighted-average exercise price, options vested and expected to vest (in usd per share) $ 6.85  
Weighted-average remaining contract life, options outstanding (in years) 4 years 4 months 17 days 5 years 1 month 13 days
Weighted-average remaining contract life, options vested and exercisable (in years) 3 years 9 months 29 days  
Weighted-average remaining contract life, options vested and expected to vest (in years) 4 years 4 months 6 days  
Aggregate intrinsic value, options outstanding $ 59,436 $ 11,049
Aggregate intrinsic value, options exercised 445  
Aggregate intrinsic value, options vested and exercisable 54,051  
Aggregate intrinsic value, options vested and expected to vest $ 59,230