Loans Held for Investment - Schedule of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023	Dec. 31, 2023
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 227.7	$ 226.1	$ 227.7	$ 220.1	$ 220.1
Provision for credit losses	9.0	11.7	14.3	26.9
Loans Charged-Off(2)	(16.3)	(14.1)	(27.3)	(23.0)
Recoveries Collected	2.8	2.7	5.4	5.4
Ending balance	232.8	224.6	232.8	224.6	227.7
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)	18.6	9.9	27.0	22.1
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Loans Charged-Off(2)			(5.7)		(7.6)
Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	4.2	2.4	4.4	3.2	3.2
Provision for credit losses	1.2	0.2	0.7	(0.9)
Loans Charged-Off(2)	0.0	0.0	0.0	0.0	0.0
Recoveries Collected	0.0	0.1	0.3	0.4
Ending balance	5.4	2.7	5.4	2.7	4.4
Residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Loans Charged-Off(2)			(0.6)		(0.6)
Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Loans Charged-Off(2)			0.0		0.0
Total real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	152.9	147.5	160.1	138.7	138.7
Provision for credit losses	(3.3)	20.4	(8.0)	33.8
Loans Charged-Off(2)	(10.1)	(10.0)	(13.3)	(14.8)
Recoveries Collected	0.2	0.8	0.9	1.0
Ending balance	139.7	158.7	139.7	158.7	160.1
Indirect consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Loans Charged-Off(2)			(4.5)		(7.3)
Direct and advance lines
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Loans Charged-Off(2)			(1.9)		(4.1)
Credit card
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Loans Charged-Off(2)			(1.4)		(2.6)
Total consumer loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	12.6	21.8	13.0	23.3	23.3
Provision for credit losses	8.6	(6.2)	11.0	(5.5)
Loans Charged-Off(2)	(4.0)	(3.3)	(7.8)	(6.7)
Recoveries Collected	1.3	1.4	2.3	2.6
Ending balance	18.5	13.7	18.5	13.7	13.0
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	58.0	54.4	50.2	54.9	54.9
Provision for credit losses	12.1	(4.5)	23.3	(5.3)
Loans Charged-Off(2)	(2.2)	(0.8)	(6.2)	(1.5)	(3.4)
Recoveries Collected	1.3	0.4	1.9	1.4
Ending balance	$ 69.2	$ 49.5	$ 69.2	$ 49.5	$ 50.2

Amount of allowance for credit loss on accounts receivable.

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Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

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Amount of expense (reversal of expense) for expected credit loss for off-balance sheet credit exposure. Excludes off-balance sheet credit exposure accounted for as insurance and instrument accounted for under derivatives and hedging.

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