Loans Held for Investment - Schedule of Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 232.8	$ 224.6	$ 227.7	$ 220.1	$ 220.1
Provision for credit losses	19.8	(0.1)	34.1	26.8
Loans Charged-Off(2)	(29.1)	(6.2)	(56.4)	(29.2)
Recoveries Collected	1.7	5.1	7.1	10.5
Ending balance	225.4	226.7	225.4	226.7	227.7
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)	20.0	3.2	47.0	25.3
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Loans Charged-Off(2)			(21.6)		(7.6)
Agricultural
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	5.4	2.7	4.4	3.2	3.2
Provision for credit losses	0.3	0.5	1.0	(0.4)
Loans Charged-Off(2)	(0.3)	0.0	(0.3)	0.0	0.0
Recoveries Collected	0.0	0.0	0.3	0.4
Ending balance	5.4	3.2	5.4	3.2	4.4
Residential
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Loans Charged-Off(2)			(0.7)		(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	139.7	158.7	160.1	138.7	138.7
Provision for credit losses	18.3	1.8	10.3	35.6
Loans Charged-Off(2)	(22.2)	(1.8)	(35.5)	(16.6)
Recoveries Collected	0.2	3.5	1.1	4.5
Ending balance	136.0	162.2	136.0	162.2	160.1
Indirect consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Loans Charged-Off(2)			(6.9)		(7.3)
Direct and advance lines
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Loans Charged-Off(2)			(2.9)		(4.1)
Credit card
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Loans Charged-Off(2)			(2.1)		(2.6)
Total consumer loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	18.5	13.7	13.0	23.3	23.3
Provision for credit losses	1.5	1.8	12.5	(3.7)
Loans Charged-Off(2)	(4.1)	(3.7)	(11.9)	(10.4)
Recoveries Collected	1.3	1.1	3.6	3.7
Ending balance	17.2	12.9	17.2	12.9	13.0
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	69.2	49.5	50.2	54.9	54.9
Provision for credit losses	(0.1)	(0.9)	23.2	(6.2)
Loans Charged-Off(2)	(2.5)	(0.7)	(8.7)	(2.2)	(3.4)
Recoveries Collected	0.2	0.5	2.1	1.9
Ending balance	$ 66.8	$ 48.4	$ 66.8	$ 48.4	$ 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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