Loans Held for Investment - Schedule of Recorded Investment in Impaired Loans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022	Jun. 30, 2023	Dec. 31, 2022	Jun. 30, 2022	Dec. 31, 2021
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	$ 0.1	$ (8.4)	$ (26.8)	$ (68.0)
Accounts Receivable, Allowance for Credit Loss	226.7	213.0	226.7	213.0	$ 224.6	$ 220.1	$ 220.4	$ 122.3
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)	3.2	4.6	25.3	60.2
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	(1.8)	(5.5)	(35.6)	(32.8)
Accounts Receivable, Allowance for Credit Loss	162.2	128.7	162.2	128.7	158.7	138.7	133.7	69.3
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	13,640.2		13,640.2			13,456.2
Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	0.9	1.6	6.2	(19.9)
Accounts Receivable, Allowance for Credit Loss	48.4	58.5	48.4	58.5	49.5	54.9	59.9	31.6
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	2,925.1		2,925.1			2,882.6
Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	(0.5)	2.5	0.4	(3.3)
Accounts Receivable, Allowance for Credit Loss	3.2	4.0	3.2	4.0	$ 2.7	3.2	6.5	0.3
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	690.5		690.5			708.3
Owner occupied
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense		3.7		(2.3)
Accounts Receivable, Allowance for Credit Loss		24.7		24.7			26.6	13.3
Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense		11.8		(0.6)
Accounts Receivable, Allowance for Credit Loss		73.7		73.7			89.5	43.9
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	8,766.2		8,766.2			8,528.6
Residential construction
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense		0.3		(0.7)
Accounts Receivable, Allowance for Credit Loss		3.1		3.1			3.4	2.4
Commercial construction
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense		(16.4)		(25.1)
Accounts Receivable, Allowance for Credit Loss		24.7		24.7			14.9	6.0
Total construction
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense		(15.9)		(25.0)
Accounts Receivable, Allowance for Credit Loss		28.5		28.5			19.1	8.9
Residential 1-4 family
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense		(0.9)		(5.4)
Accounts Receivable, Allowance for Credit Loss		19.1		19.1			18.2	13.4
Residential
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense		(1.0)		(5.5)
Accounts Receivable, Allowance for Credit Loss		20.7		20.7			19.7	14.6
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	2,212.2		2,212.2			2,188.3
Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense		(0.4)		(1.7)
Accounts Receivable, Allowance for Credit Loss		$ 5.8		$ 5.8			$ 5.4	$ 1.9
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	731.5		731.5			794.9
Total construction loans
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	1,930.3		1,930.3			1,944.4
Collateral [Member]
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	56.3		56.3			39.1
Collateral [Member] | Total real estate
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	48.7		48.7			27.2
Collateral [Member] | Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	6.9		6.9			4.6
Collateral [Member] | Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	0.7		0.7			7.3
Collateral [Member] | Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	29.6		29.6			17.5
Collateral [Member] | Residential
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	0.5		0.5			0.5
Collateral [Member] | Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	1.5		1.5			6.0
Collateral [Member] | Total construction loans
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	17.1		17.1			3.2
Business Assets [Member] | Collateral [Member]
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	5.1		5.1			7.1
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			1.9
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	4.4		4.4			3.1
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	0.7		0.7			2.1
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			1.7
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			0.2
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	50.3		50.3			32.0
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	48.7		48.7			25.3
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	1.6		1.6			1.5
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	0.0		0.0			5.2
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	29.6		29.6			15.8
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	0.5		0.5			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	1.5		1.5			5.8
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	17.1		17.1			3.2
Other Property [Member] | Collateral [Member]
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	0.9		0.9			0.0
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	0.9		0.9			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] | 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