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Stock-Based Compensation - Stock Option Activity (Details) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Number of Shares Underlying Options          
Outstanding at beginning of period (in shares)     6,307,000    
Granted (in shares)   0 186,000    
Exercised (in shares)     (583,000)    
Canceled/forfeited/expired (in shares)     (13,000)    
Outstanding at end of period (in shares) 5,897,000   5,897,000   6,307,000
Vested and expected to vest at end of period (in shares) 5,834,000   5,834,000    
Exercisable at end of period (in shares) 4,352,000   4,352,000    
Weighted Average Exercise Price          
Outstanding at beginning of period (in dollars per share)     $ 23.69    
Granted (in dollars per share)     68.10    
Exercised (in dollars per share)     24.08    
Canceled/forfeited/expired (in dollars per share)     38.57    
Outstanding at end of period (in dollars per share) $ 25.04   25.04   $ 23.69
Vested and expected to vest at end of period (in dollars per share) 24.85   24.85    
Exercisable at end of period (in dollars per share) $ 19.79   $ 19.79    
Additional disclosures          
Weighted Average Remaining Contractual Life     6 years 2 months 12 days   6 years 8 months 12 days
Weighted Average Remaining Contractual Life, Vested and expected to vest at end of period     6 years 1 month 6 days    
Weighted Average Remaining Contractual Life, Exercisable at end of period     5 years 6 months    
Aggregate Intrinsic Value for outstanding options $ 222,005   $ 222,005   $ 204,896
Exercised, Aggregate Intrinsic Value     27,515    
Vested and expected to vest, Aggregate Intrinsic Value 220,714   220,714    
Exercisable, Aggregate Intrinsic Value $ 185,936   $ 185,936    
Weighted average estimated grant date fair value of options granted (in dollars per share) $ 30.71   $ 33.02 $ 15.19  
Fair value of stock options vested $ 4,300,000 $ 4,700,000 $ 15,300,000 $ 20,400,000