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Federal and State Income Taxes Federal and State Income Taxes (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Effective Income Tax Rate Reconciliation, Amount [Abstract]      
Federal statutory tax $ 28,622 $ 44,368  
Tax exempt revenue (1,812) (3,111)  
Effect of state income taxes, net of federal benefit 3,657 2,445  
Utilization of tax credits, net of proportional amortization of low-income housing limited partnership investments (1,333) (2,087)  
Share-based compensation (1,620) (3,937)  
Adjustment to provisional amounts related to tax reform 1,895 0 $ 9,500
Other, net 1,539 425  
Total income tax expense $ 30,948 $ 38,103  
Effective Income Tax Rate Reconciliation, Percent [Abstract]      
Federal statutory tax (in hundredths) 21.00% 35.00% 35.00%
Tax exempt revenue (in hundredths) (1.30%) (2.50%)  
Effect of state income taxes, net of federal benefit (in hundredths) 2.70% 1.90%  
Utilization of tax credits, net of proportional amortization of low-income housing limited partnership investments (in hundredths) (1.00%) (1.60%)  
Share-based compensation (in hundredths) (1.20%) (3.10%)  
Adjustment to provisional amounts related to tax reform (in hundredths) 1.40% 0.00%  
Other, net (in hundredths) 1.10% 0.40%  
Total (in hundredths) 22.70% 30.10%  
Tax Cuts and Jobs Act, Revaluation of Net Deferred Tax Assets Related to Temporary Differences in Accumulated Comprehensive Income     $ 6,400
Tax Cuts and Jobs Act, Write-off of Deferred Tax Assets Related to Executive Compensation     $ 2,200