Loans Held for Investment - Schedule of Loans by Segment (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2023	Dec. 31, 2020
Loans and Leases Receivable
Loans and Leases Receivable, Deferred Income	$ (15.6)	$ (12.5)
Loans and Leases Receivable, Net of Deferred Income	18,099.2	18,279.6
Accounts Receivable, Allowance for Credit Loss	(220.1)	(227.7)	$ (144.3)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	122.3
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	(59.5)
Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(74.6)		(54.8)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	43.9
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	(37.6)
Loans and Leases Receivable, Collateral for Secured Borrowings	8,528.6	8,869.2
Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(3.6)		(1.6)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	2.4
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	0.0
Commercial Real Estate Construction [Member]
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(31.2)		(7.3)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	6.0
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	(0.2)
Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable
Loans and Leases Receivable, Collateral for Secured Borrowings	1,944.4	1,826.5
Residential Mortgage [Member]
Loans and Leases Receivable
Loans and Leases Receivable, Collateral for Secured Borrowings	2,188.3	2,244.3
Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(5.9)		(2.7)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	1.9
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	(2.3)
Loans and Leases Receivable, Collateral for Secured Borrowings	794.9	716.8
Real Estate Financing Receivable [Member]
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(138.7)	(160.1)	(80.5)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	69.3
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	(43.6)
Loans and Leases Receivable, Collateral for Secured Borrowings	13,456.2	13,656.8
Indirect consumer
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(15.3)		(16.7)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	14.3
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	0.0
Loans and Leases Receivable, Collateral for Secured Borrowings	829.7	740.9
Other consumer
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(5.2)		(4.6)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	4.6
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	0.0
Loans and Leases Receivable, Collateral for Secured Borrowings	152.9	141.6
Credit card consumer
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(2.8)		(2.6)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	2.2
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	0.0
Loans and Leases Receivable, Collateral for Secured Borrowings	75.9	76.5
Total consumer loans
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(23.3)	(13.0)	(23.9)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	21.1
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	0.0
Loans and Leases Receivable, Collateral for Secured Borrowings	1,058.5	959.0
Commercial Portfolio Segment [Member]
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(54.9)	(50.2)	(39.2)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	31.6
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	(11.4)
Loans and Leases Receivable, Collateral for Secured Borrowings	2,882.6	2,906.8
Agricultural Financing Receivable [Member]
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(3.2)	(4.4)	(0.7)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	0.3
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	(4.5)
Loans and Leases Receivable, Collateral for Secured Borrowings	708.3	769.4
Other, including overdrafts
Loans and Leases Receivable
Loans and Leases Receivable, Collateral for Secured Borrowings	9.2	0.1
Loans held for investment
Loans and Leases Receivable
Loans and Leases Receivable, Collateral for Secured Borrowings	18,114.8	$ 18,292.1
Non Owner Occupied Loans
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(27.2)		(25.5)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	17.3
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	(17.2)
Commercial Real Estate Owner Occupied
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(19.5)		(18.3)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	13.3
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	(9.5)
Commercial Real Estate Multifamily
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(27.9)		(11.0)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	13.3
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	(10.9)
Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(1.3)		(1.3)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	0.5
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	(3.4)
Construction Loans
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(36.1)		(10.2)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	8.9
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	(3.6)
Residential Real Estate 1-4 Family
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(20.5)		(11.4)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	13.4
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	(0.1)
Home Equity Loan
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(1.6)		(1.4)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	1.2
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	0.0
Residential Real Estate
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(22.1)		(12.8)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	14.6
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	(0.1)
Commercial and Floor Plans
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(49.0)		(34.2)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	27.1
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	(11.2)
Commercial Purpose Secured by 1-4 Family
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(5.7)		(4.7)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	4.4
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	(0.2)
Commercial Borrower
Loans and Leases Receivable
Accounts Receivable, Allowance for Credit Loss	(0.2)		$ (0.3)
Accounts Receivable, Allowance for Credit Loss, Beginning Balance	0.1
Financing Receivable, Allowance for Credit Losses, Effect of Change in Method	$ 0.0