Federal and State Income Taxes (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets [Abstract]
Available for sale securities mark to market	$ 24,441	$ 12,083
Share-based compensation	4,434	7,598
Credit loss allowances	49,804	58,666
Valuation adjustments	9,619	8,102
Deferred compensation	25,608	12,215
Unearned fees	9,814	9,265
Purchased loan discount	27,283	0
Other	31,812	30,859
Total deferred tax assets	182,815	138,788
Valuation Allowance [Abstract]
Valuation allowance	0	0
Deferred tax liabilities [Abstract]
Depreciation	13,901	15,817
Mortgage servicing rights	61,844	63,112
Lease financing	10,040	9,973
Acquired identifiable intangible	28,620	0
Other	32,954	34,880
Total deferred tax liabilities	147,359	123,782
Net deferred tax assets	35,456	15,006
Current income tax expense [Abstract]
Federal	103,748	141,607	$ 107,379
State	15,253	14,592	11,028
Total current income tax expense	119,001	156,199	118,407
Deferred income tax expense [Abstract]
Federal	(190)	25,525	(11,340)
State	250	869	(690)
Total deferred income tax expense	60	26,394	(12,030)
Federal and state income taxes	119,061	182,593	106,377
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Federal statutory tax	118,752	181,397	118,530
Tax exempt revenue	(8,311)	(12,402)	(10,544)
Effect of state income taxes, net of federal benefit	12,430	10,701	6,478
Utilization of tax credits, net of proportional amortization of low-income housing limited partnership investments	4,559	6,811	6,256
Share-based compensation	(2,105)	(2,817)	0
Implementation of Tax Reform Act	(1,728)	11,672	0
Deposit insurance	3,099	0	0
Other, net	1,483	853	(1,831)
Federal and state income taxes	$ 119,061	$ 182,593	$ 106,377
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory tax (in thousandths)	21.00%	35.00%	35.00%
Tax exempt revenue (in thousandths)	(1.50%)	(2.40%)	(3.10%)
Effect of state income taxes, net of federal benefit (in thousandths)	2.20%	2.00%	1.90%
Utilization of tax credits, net of proportional amortization of low-income housing limited partnership investments (in thousandths)	0.80%	1.30%	1.80%
Share-based compensation (in thousandths)	(0.40%)	(0.50%)	0.00%
Implementation of Tax Reform Act (in thousandths)	(0.30%)	2.30%	0.00%
Deposit insurance (in thousandths)	0.50%	0.00%	0.00%
Other, net (in thousandths)	0.40%	0.10%	(0.60%)
Total (in thousandths)	21.10%	35.20%	31.40%
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance as of January 1	$ 18,110	$ 15,841	$ 13,232
Additions for tax for current year positions	2,649	4,645	5,640
Settlements during the period	0	0	0
Lapses of applicable statute of limitations	(1,890)	(2,376)	(3,031)
Balance as of December 31	18,869	18,110	15,841
Unrecognized Tax Benefits that Would Impact Effective Tax Rate	12,900
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense	1,700	1,200	$ 1,000
Unrecognized Tax Benefits, Penalties and Interest Accrued	$ 5,000	$ 4,000
Unrecognized Tax Benefits, Number of Reporting Periods Open for Examination, Federal	3
Unrecognized Tax Benefits, Number of Reporting Periods Open for Examination, State, Minimum	3
Unrecognized Tax Benefits, Number of Reporting Periods Open for Examination, State, Maximum	6