Loans Held for Investment - Schedule of Recorded Investment in Impaired Loans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020	Dec. 31, 2019
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	$ 0.0	$ (5.2)	$ 5.1	$ (53.7)
Accounts Receivable, Allowance for Credit Loss	135.1	145.5	135.1	145.5	$ 135.5	$ 144.3	$ 146.1	$ 73.0
Collateral dependent borrowings	13.2		13.2			17.5
Off-Balance Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal)	0.2	4.0		52.5
Balance of defaulted loans under trouble debt restructurings	0.0		0.0
Impaired Financing Receivable, Interest Income, Accrual Method	0.0	0.0	0.0	0.0
Agriculture Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	0.0	0.0		0.0
Accounts Receivable, Allowance for Credit Loss	0.4	0.9	0.4	0.9	0.4		0.9
Total real estate loans
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	(0.7)	(3.2)	(0.1)	(21.3)
Accounts Receivable, Allowance for Credit Loss	78.9	78.6	78.9	78.6	79.4	80.5	75.2	28.9
Collateral dependent borrowings	8.5		8.5			8.9
Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	(0.1)	(1.1)	(5.5)	(23.8)
Accounts Receivable, Allowance for Credit Loss	33.9	42.0	33.9	42.0	33.7	39.2	46.5	32.6
Collateral dependent borrowings	4.0		4.0			7.8
Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Provision charged to operating expense	0.0	0.0	(0.1)	0.0
Accounts Receivable, Allowance for Credit Loss	0.4	$ 0.9	0.4	$ 0.9	$ 0.4	0.7	$ 0.9	$ 1.6
Collateral dependent borrowings	0.7		0.7			0.8
Business Assets Pledged as Collateral [Member]
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	4.3		4.3			7.4
Business Assets Pledged as Collateral [Member] | Total real estate loans
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	1.2		1.2			1.3
Business Assets Pledged as Collateral [Member] | Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	3.1		3.1			6.1
Business Assets Pledged as Collateral [Member] | Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	0.0		0.0			0.0
Real Property Pledged as Collateral [Member]
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	8.9		8.9			8.6
Real Property Pledged as Collateral [Member] | Total real estate loans
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	7.3		7.3			6.5
Real Property Pledged as Collateral [Member] | Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	0.9		0.9			1.3
Real Property Pledged as Collateral [Member] | Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	0.7		0.7			0.8
Other Property [Member]
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	0.0		0.0			1.5
Other Property [Member] | Total real estate loans
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	0.0		0.0			1.1
Other Property [Member] | Commercial
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	0.0		0.0			0.4
Other Property [Member] | Agricultural
Loans and Leases Receivable Recorded Investment, Impaired [Line Items]
Collateral dependent borrowings	$ 0.0		$ 0.0			$ 0.0