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Stock-Based Compensation - Stock Option Activity (Details) - Stock options - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock option activity, Number of Shares      
Balance at the beginning of the period (in shares) 4,045,801 3,370,901  
Options granted (in shares) 1,511,100 1,429,500  
Options forfeited (in shares) (96,856) (54,503)  
Options expired (in shares) (3,000) (1,031)  
Options exercised (in shares) (363,603) (699,066)  
Balance at the end of the period (in shares) 5,093,442 4,045,801 3,370,901
Options vested and expected to vest (in shares) 5,093,442    
Exercisable (in shares) 2,558,941 1,743,765 1,600,351
Weighted Average Exercise Price (Per Share)      
Balance at the beginning of the period (in dollars per share) $ 11.95 $ 8.50  
Options granted (in dollars per share) 22.61 15.08  
Options forfeited (in dollars per share)   12.21  
Options expired (in dollars per share)   15.69  
Options exercised (in dollars per share)   1.66  
Balance at the end of the period (in dollars per share) 15.32 11.95 $ 8.50
Options vested and expected to vest (in dollars per share) 15.32    
Exercisable (in dollars per share) $ 11.06 $ 8.87 $ 3.99
Additional information      
Weighted-Average Remaining Contractual Term, Balance outstanding 6 years 6 months 29 days 7 years 9 months 26 days 6 years 11 months 23 days
Weighted-Average Remaining Contractual Term, Options vested and expected to vest 7 years 7 months 13 days    
Weighted-Average Remaining Contractual Term, Exercisable 6 years 6 months 29 days    
Aggregate Intrinsic Value, Balance outstanding   $ 58,131  
Aggregate Intrinsic Value, Options vested and expected to vest $ 35,495    
Aggregate Intrinsic Value, Exercisable 28,017    
Intrinsic value of options exercised $ 5,700 $ 11,200 $ 5,400