Loans Held for Investment - Schedule of Recorded Investment in Impaired Loans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	$ (9.0)	$ (11.7)	$ (14.3)	$ (26.9)
Accounts Receivable, Allowance for Credit Loss	232.8	224.6	232.8	224.6	$ 227.7	$ 227.7	$ 226.1	$ 220.1
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)	18.6	9.9	27.0	22.1
Impaired Financing Receivable, Interest Income, Accrual Method	0.0	0.0	0.0	0.0
Total real estate
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	3.3	(20.4)	8.0	(33.8)
Accounts Receivable, Allowance for Credit Loss	139.7	158.7	139.7	158.7	152.9	160.1	147.5	138.7
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	13,544.3		13,544.3			13,656.8
Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	(12.1)	4.5	(23.3)	5.3
Accounts Receivable, Allowance for Credit Loss	69.2	49.5	69.2	49.5	58.0	50.2	54.4	54.9
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	3,052.9		3,052.9			2,906.8
Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	(1.2)	(0.2)	(0.7)	0.9
Accounts Receivable, Allowance for Credit Loss	5.4	$ 2.7	5.4	$ 2.7	$ 4.2	4.4	$ 2.4	$ 3.2
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	698.2		698.2			769.4
Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	9,054.5		9,054.5			8,869.2
Residential
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	2,246.4		2,246.4			2,244.3
Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	723.5		723.5			716.8
Total construction loans
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	1,519.9		1,519.9			1,826.5
Collateral [Member]
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	103.9		103.9			52.6
Collateral [Member] | Total real estate
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	39.7		39.7			45.3
Collateral [Member] | Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	63.2		63.2			6.6
Collateral [Member] | Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	1.0		1.0			0.7
Collateral [Member] | Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	27.4		27.4			26.6
Collateral [Member] | Residential
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	1.9		1.9			0.5
Collateral [Member] | Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	1.0		1.0			1.2
Collateral [Member] | Total construction loans
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	9.4		9.4			17.0
Business Assets [Member] | Collateral [Member]
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	62.4		62.4			5.2
Business Assets [Member] | Collateral [Member] | Total real estate
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	0.0		0.0			0.0
Business Assets [Member] | Collateral [Member] | Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	62.4		62.4			4.5
Business Assets [Member] | Collateral [Member] | Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	0.0		0.0			0.7
Business Assets [Member] | Collateral [Member] | Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	0.0		0.0			0.0
Business Assets [Member] | Collateral [Member] | Residential
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	0.0		0.0			0.0
Business Assets [Member] | Collateral [Member] | Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	0.0		0.0			0.0
Business Assets [Member] | Collateral [Member] | Total construction loans
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	0.0		0.0			0.0
Real Property [Member] | Collateral [Member]
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	40.7		40.7			46.7
Real Property [Member] | Collateral [Member] | Total real estate
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	39.7		39.7			45.3
Real Property [Member] | Collateral [Member] | Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	0.1		0.1			1.4
Real Property [Member] | Collateral [Member] | Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	0.9		0.9			0.0
Real Property [Member] | Collateral [Member] | Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	27.4		27.4			26.6
Real Property [Member] | Collateral [Member] | Residential
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	1.9		1.9			0.5
Real Property [Member] | Collateral [Member] | Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	1.0		1.0			1.2
Real Property [Member] | Collateral [Member] | Total construction loans
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	9.4		9.4			17.0
Other Property [Member] | Collateral [Member]
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	0.8		0.8			0.7
Other Property [Member] | Collateral [Member] | Total real estate
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	0.0		0.0			0.0
Other Property [Member] | Collateral [Member] | Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	0.7		0.7			0.7
Other Property [Member] | Collateral [Member] | Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	0.1		0.1			0.0
Other Property [Member] | Collateral [Member] | Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	0.0		0.0			0.0
Other Property [Member] | Collateral [Member] | Residential
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	0.0		0.0			0.0
Other Property [Member] | Collateral [Member] | Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	0.0		0.0			0.0
Other Property [Member] | Collateral [Member] | Total construction loans
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	$ 0.0		$ 0.0			$ 0.0