Loans Held for Investment - Schedule of Modifications to Borrowers (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2023	Jun. 30, 2023	Dec. 31, 2022
Financing Receivable, Modifications to Borrowers [Line Items]
Interest Receivable	$ 119.1	$ 119.1	$ 118.3
Borrowers in Financial Distress
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified Period, Principal Forgiveness	0.0	1.7
Financing Receivable, Excluding Accrued Interest, Modified Period	40.0	65.9
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension	$ 40.0	$ 64.2
Financing Receivable, Modification to Total Financing Receivables, Excluding Accrued Interest, Percent	0.22%	0.36%
Interest Receivable	$ 0.9	$ 0.9
Financing Receivable, Loan in Process	0.2	0.2
Borrowers in Financial Difficulty
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified Period, Principal Forgiveness	0.0	1.6
Agricultural
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Loan in Process	12.0	12.0	7.2
Agricultural | Borrowers in Financial Distress
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified Period, Principal Forgiveness		0.0
Financing Receivable, Excluding Accrued Interest, Modified Period		1.1
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension		$ 1.1
Financing Receivable, Modification to Total Financing Receivables, Excluding Accrued Interest, Percent		0.15%
Agricultural | Borrowers in Financial Difficulty
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified Period, Principal Forgiveness		$ 0.0
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension, Weighted-Average Months		7 years 2 months 12 days
Agricultural
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Loan in Process	1.8	$ 1.8	3.1
Agricultural | Borrowers in Financial Distress
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified Period, Principal Forgiveness	0.0	0.0
Financing Receivable, Excluding Accrued Interest, Modified Period	19.6	36.6
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension	$ 19.6	$ 36.6
Financing Receivable, Modification to Total Financing Receivables, Excluding Accrued Interest, Percent	2.85%	5.32%
Agricultural | Borrowers in Financial Difficulty
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified Period, Principal Forgiveness	$ 0.0	$ 0.0
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension, Weighted-Average Months	8 years 10 months 24 days	9 years 2 months 12 days
Other, including overdrafts
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Loan in Process	$ 0.0	$ 0.0	0.0
Commercial
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Loan in Process	4.2	4.2	7.5
Commercial | Borrowers in Financial Distress
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified Period, Principal Forgiveness	0.0	1.6
Financing Receivable, Excluding Accrued Interest, Modified Period	13.9	19.0
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension	$ 13.9	$ 17.4
Financing Receivable, Modification to Total Financing Receivables, Excluding Accrued Interest, Percent	0.16%	0.22%
Commercial | Borrowers in Financial Difficulty
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified Period, Principal Forgiveness	$ 0.0	$ 1.3
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension, Weighted-Average Months	6 years 4 months 24 days	6 years 4 months 24 days
Total construction loans
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Loan in Process	$ 14.5	$ 14.5	11.6
Total construction loans | Borrowers in Financial Distress
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified Period, Principal Forgiveness		0.0
Financing Receivable, Excluding Accrued Interest, Modified Period		0.1
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension		$ 0.1
Financing Receivable, Modification to Total Financing Receivables, Excluding Accrued Interest, Percent		0.01%
Total construction loans | Borrowers in Financial Difficulty
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified Period, Principal Forgiveness		$ 0.0
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension, Weighted-Average Months		12 years 4 months 24 days
Residential
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Loan in Process	3.9	$ 3.9	13.2
Residential | Borrowers in Financial Distress
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified Period, Principal Forgiveness	0.0	0.1
Financing Receivable, Excluding Accrued Interest, Modified Period	0.1	0.3
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension	$ 0.1	$ 0.2
Financing Receivable, Modification to Total Financing Receivables, Excluding Accrued Interest, Percent	0.00%	0.01%
Residential | Borrowers in Financial Difficulty
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified Period, Principal Forgiveness	$ 0.0	$ 0.3
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension, Weighted-Average Months	16 years 2 months 12 days	19 years 6 months
Total real estate
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Loan in Process	$ 34.6	$ 34.6	39.5
Total real estate | Borrowers in Financial Distress
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified Period, Principal Forgiveness	0.0	1.7
Financing Receivable, Excluding Accrued Interest, Modified Period	14.0	20.5
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension	$ 14.0	$ 18.8
Financing Receivable, Modification to Total Financing Receivables, Excluding Accrued Interest, Percent	0.10%	0.15%
Total real estate | Borrowers in Financial Difficulty
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified Period, Principal Forgiveness	$ 0.0	$ 1.6
Commercial
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Loan in Process	8.0	8.0	10.9
Commercial | Borrowers in Financial Distress
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified Period, Principal Forgiveness	0.0	0.0
Financing Receivable, Excluding Accrued Interest, Modified Period	6.4	8.8
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension	$ 6.4	$ 8.8
Financing Receivable, Modification to Total Financing Receivables, Excluding Accrued Interest, Percent	0.21%	0.29%
Commercial | Borrowers in Financial Difficulty
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified Period, Principal Forgiveness	$ 0.0	$ 0.0
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension, Weighted-Average Months	8 years 3 months 18 days	9 years
Indirect consumer
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Loan in Process	$ 9.1	$ 9.1	12.3
Indirect consumer | Borrowers in Financial Difficulty
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified Period, Principal Forgiveness	$ 0.0	$ 0.0
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension, Weighted-Average Months	7 years	8 years 3 months 18 days
Direct and advance lines
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Loan in Process	$ 1.2	$ 1.2	1.2
Direct and advance lines | Borrowers in Financial Difficulty
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified Period, Principal Forgiveness	$ 0.0	$ 0.0
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension, Weighted-Average Months	8 years	8 years
Credit Card Receivable [Member]
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Loan in Process	$ 1.5	$ 1.5	1.8
Total consumer loans
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Loan in Process	11.8	11.8	$ 15.3
Total consumer loans | Borrowers in Financial Difficulty
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Excluding Accrued Interest, Modified Period, Principal Forgiveness	$ 0.0	$ 0.0