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Income Taxes (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Mar. 31, 2017
Jun. 30, 2018
Jun. 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 provision $ 26,529 $ 33,019   $ 47,162 $ 59,865  
Effective income tax rates 27.80% 37.50%   27.70% 37.70%  
Actual income tax rates 27.20% 36.80%   27.10% 35.30%  
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         $ 300  
Interest expense related to unrecognized income tax benefits 100 $ 100   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