Loans Held for Investment - Schedule of Recorded Investment in Impaired Loans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022	Dec. 31, 2021
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	$ (11.7)	$ 1.7	$ (26.9)	$ (59.6)
Accounts Receivable, Allowance for Credit Loss	224.6	220.4	224.6	220.4	$ 226.1	$ 220.1	$ 247.2	$ 122.3
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)	9.9	(1.7)	22.1	55.6
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	(20.4)	11.2	(33.8)	(27.3)
Accounts Receivable, Allowance for Credit Loss	158.7	133.7	158.7	133.7	147.5	138.7	169.6	69.3
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	13,604.4		13,604.4			13,456.2
Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	4.5	(2.4)	5.3	(21.5)
Accounts Receivable, Allowance for Credit Loss	49.5	59.9	49.5	59.9	54.4	54.9	58.2	31.6
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	3,002.7		3,002.7			2,882.6
Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	(0.2)	(6.3)	0.9	(5.8)
Accounts Receivable, Allowance for Credit Loss	2.7	6.5	2.7	6.5	$ 2.4	3.2	(1.0)	0.3
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	688.0		688.0			708.3
Owner occupied
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense		(1.4)		(6.0)
Accounts Receivable, Allowance for Credit Loss		26.6		26.6			25.2	13.3
Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense		10.3		(12.4)
Accounts Receivable, Allowance for Credit Loss		89.5		89.5			125.0	43.9
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	8,813.9		8,813.9			8,528.6
Residential construction
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense		(0.1)		(1.0)
Accounts Receivable, Allowance for Credit Loss		3.4		3.4			3.3	2.4
Commercial construction
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense		(4.7)		(8.7)
Accounts Receivable, Allowance for Credit Loss		14.9		14.9			10.2	6.0
Total construction
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense		(5.0)		(9.1)
Accounts Receivable, Allowance for Credit Loss		19.1		19.1			14.0	8.9
Residential 1-4 family
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense		0.7		(4.5)
Accounts Receivable, Allowance for Credit Loss		18.2		18.2			18.7	13.4
Residential
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense		0.8		(4.5)
Accounts Receivable, Allowance for Credit Loss		19.7		19.7			20.1	14.6
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	2,198.3		2,198.3			2,188.3
Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense		5.1		(1.3)
Accounts Receivable, Allowance for Credit Loss		$ 5.4		$ 5.4			$ 10.5	$ 1.9
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	755.7		755.7			794.9
Total construction loans
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	1,836.5		1,836.5			1,944.4
Collateral [Member]
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss	63.5		63.5			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	55.9		55.9			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	33.9		33.9			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	3.7		3.7			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	17.8		17.8			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	3.2		3.2			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	1.6		1.6			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	1.4		1.4			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	0.2		0.2			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	1.6		1.6			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	60.3		60.3			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	54.3		54.3			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	5.5		5.5			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	0.5		0.5			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	32.3		32.3			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	3.7		3.7			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	$ 17.8		$ 17.8			$ 3.2