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Summary of Stock Option Activity (Details) - Stock Options [Member] - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]    
Outstanding Options, Aggregate Intrinsic Value, beginning of period $ 102,625 $ 107,273
Outstanding Options Exercisable, Aggregate Intrinsic Value 101,049  
Outstanding Options Fully vested and expected to vest, Aggregate Intrinsic Value $ 102,625  
Non-vested Options, Shares [Roll Forward]    
Non-vested Shares, beginning of the period (in shares) 256,875  
Non-vested Shares, Vested (in shares) (206,875)  
Non-vested Shares, end of the period (in shares) 50,000 256,875
Non-vested Shares, Weighted-Average Grant-Date Fair Value [Roll Forward]    
Non-vested shares, Weighted Average Grant-Date Fair Value, beginning of the period (in dollars per share) $ 31.75  
Non-vested shares Vested, Weighted Average Grant-Date Fair Value (in dollars per share) 31.82  
Non-vested shares, Weighted Average Grant-Date Fair Value, end of the period (in dollars per share) $ 31.45 $ 31.75
Outstanding Options Weighted-Average Remaining Contractual Term 4 years 18 days 5 years 21 days
Outstanding Options Exercisable, Weighted-Average Remaining Contractual Term 4 years  
Outstanding Options Fully vested and expected to vest Weighted-Average Remaining Contractual Term 4 years 18 days  
Outstanding Options, Shares [Roll Forward]    
Outstanding as of December 31, 2017 1,960,459  
Options Exercised (in shares) (141,660)  
Options Forfeited/cancelled (in shares) (9,531)  
Outstanding Options at end of the period (in shares) 1,809,268 1,960,459
Outstanding Options Exercisable as of December 31, 2018 (in shares) 1,759,268  
Outstanding Options Fully vested and expected to vest as of December 31, 2018 (in shares) 1,809,268  
Outstanding Options, Weighted Average Exercise Price [Roll Forward]    
Outstanding Options Weighted-Average Exercise Price, beginning of period (in dollars per share) $ 23.48  
Options Exercised Weighted-Average Exercise Price (in dollars per share) 42.66  
Options Forfeited/cancelled Weighted-Average Exercise Price (in dollars per share) 35.19  
Outstanding Options Weighted-Average Exercise Price, end of period (in dollars per share) 21.92 $ 23.48
Outstanding Options Exercisable, Weighted-Average Exercise Price (in dollars per share) 21.20  
Outstanding Options Fully vested and expected to vest Weighted-Average Exercise Price (in dollars per share) $ 21.92