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Employee Benefits and Share-Based Compensation - Stock Option Activity (Details) - 2009 Plan - Employee stock option - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Number of Shares      
Outstanding at beginning of period (in shares) 3,748    
Granted (in shares) 292    
Exercised (in shares) (379)    
Expired (in shares) 0    
Forfeited (in shares) (117)    
Outstanding at end of period (in shares) 3,544   3,748
Exercisable at end of period (in shares) 1,275    
Vested and expected to vest at end of period (in shares) 3,544    
Weighted Average Exercise Price      
Outstanding beginning balance (in dollars per share) $ 41.27    
Granted (in dollars per share) 76.18    
Exercised (in dollars per share) 31.83    
Expired (in dollars per share) 0.00    
Forfeited (in dollars per share) 44.18    
Outstanding ending balance (in dollars per share) 45.06   $ 41.27
Exercisable at end of period (in dollars per share) 31.24    
Vested and expected to vest at end of period (in dollars per share) $ 45.06    
Weighted-average remaining years, outstanding 7 years 11 months 1 day   7 years 7 months 13 days
Weighted-average remaining years, exercisable at end of period 6 years 3 months 7 days    
Weighted-average remaining vesting period ( in years ) 7 years 11 months 1 day    
Aggregate Intrinsic Value      
Aggregate intrinsic value, outstanding at end of period $ 127,007   $ 78,365
Aggregate intrinsic value, exercisable at end of period 63,241    
Aggregate intrinsic value, vested and expected to vest at end of period $ 127,007    
Weighted-average fair value (in dollars per share) $ 24.07 $ 14.22  
Intrinsic value of options exercised $ 17,100 $ 2,000  
Unrecognized compensation cost of unvested stock options $ 32,500    
Weighted average period of compensation cost not yet recognized (in years) 2 years 10 months 10 days