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Stock-Based Compensation - Stock Option Activity (Details) - Stock options - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Stock option activity, Number of Shares      
Balance at the beginning of the period (in shares) 5,093,442 4,045,801  
Options granted (in shares) 1,805,937 1,511,100  
Options forfeited (in shares) (107,720) (96,856)  
Options expired (in shares)   (3,000)  
Options exercised (in shares) (824,731) (363,603)  
Balance at the end of the period (in shares) 5,966,928 5,093,442 4,045,801
Options vested and expected to vest (in shares) 5,966,928    
Exercisable (in shares) 3,058,659 2,558,941 1,743,765
Weighted Average Exercise Price (Per Share)      
Balance at the beginning of the period (in dollars per share) $ 15.32 $ 11.95  
Options granted (in dollars per share) 27.43 22.61  
Options forfeited (in dollars per share) 21.66 17.08  
Options expired (in dollars per share)   21.99  
Options exercised (in dollars per share) 9.24 7.69  
Balance at the end of the period (in dollars per share) 19.71 15.32 $ 11.95
Options vested and expected to vest (in dollars per share) 19.71    
Exercisable (in dollars per share) $ 15.12 $ 11.06 $ 8.87
Additional information      
Weighted-Average Remaining Contractual Term, Balance outstanding 7 years 6 months 4 days 7 years 7 months 13 days 7 years 9 months 26 days
Weighted-Average Remaining Contractual Term, Options vested and expected to vest 7 years 6 months 4 days    
Weighted-Average Remaining Contractual Term, Exercisable 6 years 5 months 1 day    
Aggregate Intrinsic Value, Balance outstanding $ 99,273 $ 35,495 $ 58,131
Aggregate Intrinsic Value, Options vested and expected to vest 99,273    
Aggregate Intrinsic Value, Exercisable 64,417    
Intrinsic value of options exercised $ 23,600 $ 5,700 $ 11,200