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Income Taxes (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Mar. 31, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Operating Loss Carryforwards [Line Items]            
Federal tax rate, percent       21.00%   35.00%
Income tax benefit from the Tax Act           $ 39,300
Income tax expense, foreign earnings $ 100          
Income tax provision $ 29,711 $ 38,608   $ 76,873 $ 98,473  
Effective income tax rates 27.70% 37.10%   27.70% 37.40%  
Actual income tax rates 27.10% 37.30%   27.10% 36.00%  
Unrecognized income tax benefits $ 800     $ 800   800
Accrued interest expense related to unrecognized income tax benefits 100     100   100
Interest income related to unrecognized income tax benefits 100     100 $ 300  
Interest expense related to unrecognized income tax benefits   $ 100        
Unrecognized tax benefits, decrease resulting from settlements with taxing authorities     $ 3,300      
Other Long-term Liabilities [Member]            
Operating Loss Carryforwards [Line Items]            
Income tax reserves $ 900     $ 900   $ 900