Loans - Schedule of Recorded Investment in Past Due Loans by Class (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Loans and Leases Receivable Recorded Investment, Past Due
Total loans	$ 9,331.7	$ 9,807.5
Impaired Financing Receivable, Interest Income, Accrual Method	0.0	0.3
Financing Receivable, Allowance for Credit Loss, Writeoff	16.6	20.9
Loans and Leases Receivable, Nonperforming, Accrual of Interest	0.0
Financing Receivable, Accrued Interest, Writeoff	0.0
Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Non-accrual Loans	8.0	9.6
Loans held for investment	3,971.5	3,743.2
Financing Receivable, Allowance for Credit Loss, Writeoff	2.3	0.4
Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Non-accrual Loans	0.7	0.7
Loans held for investment	247.8	265.0
Total loans	247.8	265.0
Financing Receivable, Allowance for Credit Loss, Writeoff	1.2	0.5
Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Non-accrual Loans	0.0	1.1
Loans held for investment	262.0	250.9
Total loans	262.0	250.9
Financing Receivable, Allowance for Credit Loss, Writeoff	0.1	0.0
Commercial Real Estate Construction [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Non-accrual Loans	0.0	0.1
Loans held for investment	498.0	523.5
Total loans	498.0	523.5
Financing Receivable, Allowance for Credit Loss, Writeoff	0.1	0.0
Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Non-accrual Loans	0.7	1.9
Loans held for investment	1,007.8	1,039.4
Residential Mortgage [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Non-accrual Loans	2.9	4.6
Loans held for investment	1,538.2	1,396.3
Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Non-accrual Loans	4.9	6.2
Loans held for investment	213.9	220.6
Total loans	213.9	220.6
Financing Receivable, Allowance for Credit Loss, Writeoff	0.7	0.0
Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Non-accrual Loans	16.5	22.3
Loans held for investment	6,731.4	6,399.5
Impaired Financing Receivable, Interest Income, Accrual Method		0.1
Financing Receivable, Allowance for Credit Loss, Writeoff	4.5	0.9
Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due
Non-accrual Loans	1.7	1.9
Loans held for investment	737.6	805.1
Total loans	737.6	805.1
Financing Receivable, Allowance for Credit Loss, Writeoff	3.5	4.1
Other consumer
Loans and Leases Receivable Recorded Investment, Past Due
Non-accrual Loans	0.1	0.4
Loans held for investment	129.2	150.6
Total loans	129.2	150.6
Financing Receivable, Allowance for Credit Loss, Writeoff	2.9	3.9
Credit card consumer
Loans and Leases Receivable Recorded Investment, Past Due
Non-accrual Loans	0.0	0.0
Loans held for investment	64.9	70.2
Financing Receivable, Allowance for Credit Loss, Writeoff	1.8	2.8
Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due
Non-accrual Loans	1.8	2.3
Loans held for investment	931.7	1,025.9
Financing Receivable, Allowance for Credit Loss, Writeoff	8.2	10.8
Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Non-accrual Loans	5.0	11.8
Loans held for investment	1,475.5	2,153.9
Impaired Financing Receivable, Interest Income, Accrual Method		0.2
Financing Receivable, Allowance for Credit Loss, Writeoff	3.7	9.1
Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due
Non-accrual Loans	0.0	0.0
Loans held for investment	1.5	1.6
Loans held for investment
Loans and Leases Receivable Recorded Investment, Past Due
Non-accrual Loans	24.9	39.5
Loans held for investment	9,344.0	9,828.5
Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Non-accrual Loans	1.6	3.1
Loans held for investment	203.9	247.6
Financing Receivable, Allowance for Credit Loss, Writeoff	0.2	0.1
Financing Receivables, 30 to 59 Days Past Due | Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	1.1	7.6
Financing Receivables, 30 to 59 Days Past Due | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.2	2.5
Financing Receivables, 30 to 59 Days Past Due | Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	4.2	1.5
Financing Receivables, 30 to 59 Days Past Due | Commercial Real Estate Construction [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.0	12.2
Financing Receivables, 30 to 59 Days Past Due | Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	4.4	16.2
Financing Receivables, 30 to 59 Days Past Due | Residential Mortgage [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	3.0	4.7
Financing Receivables, 30 to 59 Days Past Due | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	1.9	2.0
Financing Receivables, 30 to 59 Days Past Due | Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	10.4	30.5
Financing Receivables, 30 to 59 Days Past Due | Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	5.1	6.4
Financing Receivables, 30 to 59 Days Past Due | Other consumer
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.5	0.8
Financing Receivables, 30 to 59 Days Past Due | Credit card consumer
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.6	0.6
Financing Receivables, 30 to 59 Days Past Due | Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	6.2	7.8
Financing Receivables, 30 to 59 Days Past Due | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	4.9	6.2
Financing Receivables, 30 to 59 Days Past Due | Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.0	0.0
Financing Receivables, 30 to 59 Days Past Due | Loans held for investment
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	22.2	44.9
Financing Receivables, 30 to 59 Days Past Due | Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.7	0.4
Financing Receivables, 60 to 89 Days Past Due | Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	1.0	1.2
Financing Receivables, 60 to 89 Days Past Due | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.0	1.1
Financing Receivables, 60 to 89 Days Past Due | Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.0	0.4
Financing Receivables, 60 to 89 Days Past Due | Commercial Real Estate Construction [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.0	0.0
Financing Receivables, 60 to 89 Days Past Due | Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.0	1.5
Financing Receivables, 60 to 89 Days Past Due | Residential Mortgage [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.8	1.6
Financing Receivables, 60 to 89 Days Past Due | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.2	0.0
Financing Receivables, 60 to 89 Days Past Due | Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	2.0	4.3
Financing Receivables, 60 to 89 Days Past Due | Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	1.4	2.0
Financing Receivables, 60 to 89 Days Past Due | Other consumer
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.2	0.2
Financing Receivables, 60 to 89 Days Past Due | Credit card consumer
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.2	0.4
Financing Receivables, 60 to 89 Days Past Due | Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	1.8	2.6
Financing Receivables, 60 to 89 Days Past Due | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.7	1.8
Financing Receivables, 60 to 89 Days Past Due | Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.0	0.0
Financing Receivables, 60 to 89 Days Past Due | Loans held for investment
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	4.5	9.3
Financing Receivables, 60 to 89 Days Past Due | Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.0	0.6
Financing Receivables, Equal to Greater than 90 Days Past Due | Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.6	4.0
Financing Receivables, Equal to Greater than 90 Days Past Due | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.0	0.1
Financing Receivables, Equal to Greater than 90 Days Past Due | Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.0	0.0
Financing Receivables, Equal to Greater than 90 Days Past Due | Commercial Real Estate Construction [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.0	0.0
Financing Receivables, Equal to Greater than 90 Days Past Due | Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.0	0.1
Financing Receivables, Equal to Greater than 90 Days Past Due | Residential Mortgage [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.1	0.5
Financing Receivables, Equal to Greater than 90 Days Past Due | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.0	0.0
Financing Receivables, Equal to Greater than 90 Days Past Due | Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.7	4.6
Financing Receivables, Equal to Greater than 90 Days Past Due | Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.4	0.5
Financing Receivables, Equal to Greater than 90 Days Past Due | Other consumer
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.1	0.2
Financing Receivables, Equal to Greater than 90 Days Past Due | Credit card consumer
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.5	0.6
Financing Receivables, Equal to Greater than 90 Days Past Due | Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	1.0	1.3
Financing Receivables, Equal to Greater than 90 Days Past Due | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	1.1	1.2
Financing Receivables, Equal to Greater than 90 Days Past Due | Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.0	0.0
Financing Receivables, Equal to Greater than 90 Days Past Due | Loans held for investment
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	2.8	8.5
Financing Receivables, Equal to Greater than 90 Days Past Due | Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.0	1.4
Financial Asset, Past Due | Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	2.7	12.8
Financial Asset, Past Due | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.2	3.7
Financial Asset, Past Due | Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	4.2	1.9
Financial Asset, Past Due | Commercial Real Estate Construction [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.0	12.2
Financial Asset, Past Due | Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	4.4	17.8
Financial Asset, Past Due | Residential Mortgage [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	3.9	6.8
Financial Asset, Past Due | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	2.1	2.0
Financial Asset, Past Due | Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	13.1	39.4
Financial Asset, Past Due | Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	6.9	8.9
Financial Asset, Past Due | Other consumer
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.8	1.2
Financial Asset, Past Due | Credit card consumer
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	1.3	1.6
Financial Asset, Past Due | Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	9.0	11.7
Financial Asset, Past Due | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	6.7	9.2
Financial Asset, Past Due | Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.0	0.0
Financial Asset, Past Due | Loans held for investment
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	29.5	62.7
Financial Asset, Past Due | Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	0.7	2.4
Financial Asset, Not Past Due | Commercial Real Estate Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	3,960.8	3,720.8
Financial Asset, Not Past Due | Land Acquisition And Development Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	246.9	260.6
Financial Asset, Not Past Due | Residential Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	257.8	247.9
Financial Asset, Not Past Due | Commercial Real Estate Construction [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	498.0	511.2
Financial Asset, Not Past Due | Real Estate Construction Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	1,002.7	1,019.7
Financial Asset, Not Past Due | Residential Mortgage [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	1,531.4	1,384.9
Financial Asset, Not Past Due | Agricultural Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	206.9	212.4
Financial Asset, Not Past Due | Real Estate Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	6,701.8	6,337.8
Financial Asset, Not Past Due | Indirect consumer
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	729.0	794.3
Financial Asset, Not Past Due | Other consumer
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	128.3	149.0
Financial Asset, Not Past Due | Credit card consumer
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	63.6	68.6
Financial Asset, Not Past Due | Total consumer loans
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	920.9	1,011.9
Financial Asset, Not Past Due | Commercial Portfolio Segment [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	1,463.8	2,132.9
Financial Asset, Not Past Due | Other, including overdrafts
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	1.5	1.6
Financial Asset, Not Past Due | Loans held for investment
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	9,289.6	9,726.3
Financial Asset, Not Past Due | Agricultural Financing Receivable [Member]
Loans and Leases Receivable Recorded Investment, Past Due
Financing Receivable, Loan in Process	$ 201.6	$ 242.1