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Federal and State Income Taxes Federal and State Income Taxes (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Effective Income Tax Rate Reconciliation, Amount [Abstract]          
Federal statutory tax $ 31,182 $ 47,800 $ 59,804 $ 92,168  
Tax exempt revenue (1,653) (3,224) (3,465) (6,335)  
Effect of state income taxes, net of federal benefit 3,288 2,944 6,945 5,389  
Utilization of tax credits, net of proportional amortization of low-income housing limited partnership investments (1,334) (889) (2,667) (2,976)  
Share-based compensation (424) 1,636 (2,044) (2,301)  
Adjustment to provisional amounts related to tax reform 0 0 1,895 0  
Other, net 2,271 (562) 3,810 (137)  
Total income tax expense $ 33,330 $ 47,705 $ 64,278 $ 85,808  
Effective Income Tax Rate Reconciliation, Percent [Abstract]          
Federal statutory tax (in hundredths) 21.00% 35.00% 21.00% 35.00% 35.00%
Tax exempt revenue (in hundredths) (1.10%) (2.40%) (1.20%) (2.40%)  
Effect of state income taxes, net of federal benefit (in hundredths) 2.20% 2.20% 2.40% 2.00%  
Utilization of tax credits, net of proportional amortization of low-income housing limited partnership investments (in hundredths) (0.90%) (0.70%) (0.90%) (1.10%)  
Share-based compensation (in hundredths) (0.30%) 1.20% (0.70%) (0.90%)  
Adjustment to provisional amounts related to tax reform (in hundredths) 0.00% 0.00% 0.70% 0.00%  
Other, net (in hundredths) 1.50% (0.40%) 1.30% 0.00%  
Total (in hundredths) 22.40% 34.90% 22.60% 32.60%