Loans Held for Investment - Schedule of Modifications to Borrowers (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2023	Dec. 31, 2022
Financing Receivable, Modifications to Borrowers [Line Items]
Interest Receivable	$ 129.2	$ 129.2	$ 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	33.2	91.2
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension	$ 32.6	$ 88.9
Financing Receivable, Modification to Total Financing Receivables, Excluding Accrued Interest, Percent	0.18%	0.50%
Interest Receivable	$ 1.7	$ 1.7
Financing Receivable, Loan in Process	0.1	0.1
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	1.9	1.9	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	0.0
Financing Receivable, Excluding Accrued Interest, Modified Period	6.1	6.4
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension	$ 6.1	$ 6.4
Financing Receivable, Modification to Total Financing Receivables, Excluding Accrued Interest, Percent	0.83%	0.87%
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	4 years	4 years 7 months 6 days
Agricultural
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Loan in Process	$ 1.0	$ 1.0	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	0.5	36.4
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension	$ 0.5	$ 36.4
Financing Receivable, Modification to Total Financing Receivables, Excluding Accrued Interest, Percent	0.07%	5.27%
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	3 years 6 months	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	1.6	1.6	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	11.5	25.2
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension	$ 10.9	$ 23.0
Financing Receivable, Modification to Total Financing Receivables, Excluding Accrued Interest, Percent	0.13%	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	$ 1.3
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension, Weighted-Average Months	4 years 7 months 6 days	5 years 8 months 12 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	0.0
Financing Receivable, Excluding Accrued Interest, Modified Period	13.1	13.2
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension	$ 13.1	$ 13.2
Financing Receivable, Modification to Total Financing Receivables, Excluding Accrued Interest, Percent	0.68%	0.68%
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	$ 0.0
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension, Weighted-Average Months	9 years	9 years
Residential
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Loan in Process	$ 5.2	$ 5.2	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	0.4	0.6
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension	$ 0.4	$ 0.5
Financing Receivable, Modification to Total Financing Receivables, Excluding Accrued Interest, Percent	0.02%	0.03%
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	144 years 4 months 24 days	139 years 2 months 12 days
Total real estate
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Loan in Process	$ 23.2	$ 23.2	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	31.1	45.4
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension	$ 30.5	$ 43.1
Financing Receivable, Modification to Total Financing Receivables, Excluding Accrued Interest, Percent	0.23%	0.33%
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	18.9	18.9	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	1.6	9.4
Financing Receivable, Excluding Accrued Interest, Modified Period, Term Extension	$ 1.6	$ 9.4
Financing Receivable, Modification to Total Financing Receivables, Excluding Accrued Interest, Percent	0.05%	0.32%
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	7 years 9 months 18 days	8 years 9 months 18 days
Indirect consumer
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Loan in Process	$ 9.0	$ 9.0	12.3
Direct and advance lines
Financing Receivable, Modifications to Borrowers [Line Items]
Financing Receivable, Loan in Process	0.8	0.8	1.2
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	$ 11.3	$ 11.3	$ 15.3