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Accounting for Stock-based Compensation - Outstanding Stock Option Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Number of Shares, Outstanding Ending Balance 201,810    
Number of Shares, Exercisable at December 31, 2018 201,810    
Weighted Average Exercise Price, Outstanding Ending Balance $ 33.68    
Weighted Average Exercise Price, Exercisable at December 31, 2018 $ 33.68    
Employee Stock Option      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Number of Shares, Outstanding Beginning Balance 411,498 587,407 723,005
Number of Shares, Exercised (209,688) (175,909) (128,301)
Number of Shares, Granted 0 0 0
Number of Shares, Forfeited/Expired     (7,297)
Number of Shares, Outstanding Ending Balance 201,810 411,498 587,407
Number of Shares, Vested plus expected to vest at December 31, 2018 201,810    
Number of Shares, Exercisable at December 31, 2018 201,810    
Weighted Average Exercise Price, Outstanding Beginning Balance $ 30.71 $ 29.56 $ 28.62
Weighted Average Exercise Price, Exercised 27.86 26.84 23.65
Weighted Average Exercise Price, Forfeited/Expired     40.68
Weighted Average Exercise Price, Outstanding Ending Balance 33.68 $ 30.71 $ 29.56
Weighted Average Exercise Price, Vested plus expected to vest at December 31, 2018 33.68    
Weighted Average Exercise Price, Exercisable at December 31, 2018 $ 33.68    
Aggregate Intrinsic Value, Outstanding at December 31,2018 $ 6,276,432    
Aggregate Intrinsic Value, Vested plus expected to vest at December 31, 2018 6,276,432    
Aggregate Intrinsic Value, Exercisable at December 31, 2018 $ 6,276,432