Federal and State Income Taxes Federal and State Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Federal statutory tax	$ 31,963	$ 44,880	$ 91,767	$ 137,048
Tax exempt revenue	(2,059)	(3,001)	(5,524)	(9,336)
Effect of state income taxes, net of federal benefit	3,740	2,486	10,685	7,875
Utilization of tax credits, net of proportional amortization of low-income housing limited partnership investments	56	(387)	(2,611)	(3,363)
Effective Income Tax Rate Reconciliation, Share-based Compensation, Excess Tax Benefit, Amount	(26)	(169)	(2,070)	(2,470)
Adjustment to provisional amounts related to tax reform	0	0	1,895	0
Other, net	988	(1,371)	4,798	(1,508)
Total income tax expense	$ 34,662	$ 42,438	$ 98,940	$ 128,246
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)	(1.40%)	(2.30%)	(1.30%)	(2.40%)
Effect of state income taxes, net of federal benefit (in hundredths)	2.50%	1.90%	2.40%	2.00%
Utilization of tax credits, net of proportional amortization of low-income housing limited partnership investments (in hundredths)	0.00%	(0.30%)	(0.60%)	(0.90%)
Share-based compensation (in hundredths)	0.00%	(0.10%)	(0.50%)	(0.60%)
Adjustment to provisional amounts related to tax reform (in hundredths)	0.00%	0.00%	0.40%	0.00%
Other, net (in hundredths)	0.70%	(1.10%)	1.20%	(0.30%)
Total (in hundredths)	22.80%	33.10%	22.60%	32.80%