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Loans Held for Investment - Schedule of Loans by Segment (Details) - USD ($) $ in Millions | Jun. 30, 2023 | Mar. 31, 2023 | Dec. 31, 2022 | Jun. 30, 2022 | Mar. 31, 2022 | Dec. 31, 2021
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Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Deferred loan fees and costs | $ (13.6) |  | $ (15.6) |  |  |
Loans held for investment, net of deferred fees and costs | 18,263.4 |  | 18,099.2 |  |  |
Allowance for credit losses | (224.6) | $ (226.1) | (220.1) | $ (220.4) | $ (247.2) | $ (122.3)
Net loans held for investment | 18,038.8 |  | 17,879.1 |  |  |
Nonaccrual loans | 33.2 |  | 39.8 |  |  |
Commercial real estate |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Allowance for credit losses |  |  |  | (89.5) | (125.0) | (43.9)
Financing Receivable, Loan in Process | 4.2 |  | 7.5 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 8,813.9 |  | 8,528.6 |  |  |
Nonaccrual loans | 35.4 |  | 19.6 |  |  |
Commercial real estate | 30 to 59 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 4.1 |  | 5.6 |  |  |
Commercial real estate | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.1 |  | 0.8 |  |  |
Commercial real estate | Equal to or Greater than 90 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.0 |  | 1.1 |  |  |
Commercial real estate | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 8,774.3 |  | 8,501.5 |  |  |
Residential |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Allowance for credit losses |  |  |  | (3.4) | (3.3) | (2.4)
Commercial |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Allowance for credit losses |  |  |  | (14.9) | (10.2) | (6.0)
Total construction loans |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 14.5 |  | 11.6 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 1,836.5 |  | 1,944.4 |  |  |
Nonaccrual loans | 18.2 |  | 3.7 |  |  |
Total construction loans | 30 to 59 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 8.2 |  | 10.4 |  |  |
Total construction loans | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 3.0 |  | 0.6 |  |  |
Total construction loans | Equal to or Greater than 90 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 3.3 |  | 0.6 |  |  |
Total construction loans | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 1,803.8 |  | 1,929.1 |  |  |
Residential |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Allowance for credit losses |  |  |  | (19.7) | (20.1) | (14.6)
Financing Receivable, Loan in Process | 3.9 |  | 13.2 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 2,198.3 |  | 2,188.3 |  |  |
Nonaccrual loans | 7.7 |  | 6.4 |  |  |
Residential | 30 to 59 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 1.2 |  | 9.9 |  |  |
Residential | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 2.1 |  | 2.1 |  |  |
Residential | Equal to or Greater than 90 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.6 |  | 1.2 |  |  |
Residential | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 2,186.7 |  | 2,168.7 |  |  |
Agricultural |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Allowance for credit losses |  |  |  | (5.4) | (10.5) | (1.9)
Financing Receivable, Loan in Process | 12.0 |  | 7.2 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 755.7 |  | 794.9 |  |  |
Nonaccrual loans | 6.8 |  | 7.6 |  |  |
Agricultural | 30 to 59 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 7.8 |  | 1.1 |  |  |
Agricultural | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 3.9 |  | 6.1 |  |  |
Agricultural | Equal to or Greater than 90 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.3 |  | 0.0 |  |  |
Agricultural | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 736.9 |  | 780.1 |  |  |
Total real estate |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Allowance for credit losses | (158.7) | (147.5) | (138.7) | (133.7) | (169.6) | (69.3)
Financing Receivable, Loan in Process | 34.6 |  | 39.5 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 13,604.4 |  | 13,456.2 |  |  |
Nonaccrual loans | 68.1 |  | 37.3 |  |  |
Total real estate | 30 to 59 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 21.3 |  | 27.0 |  |  |
Total real estate | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 9.1 |  | 9.6 |  |  |
Total real estate | Equal to or Greater than 90 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 4.2 |  | 2.9 |  |  |
Total real estate | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 13,501.7 |  | 13,379.4 |  |  |
Indirect consumer |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Allowance for credit losses |  |  |  | (13.1) | (13.5) | (14.3)
Financing Receivable, Loan in Process | 9.1 |  | 12.3 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 764.1 |  | 829.7 |  |  |
Nonaccrual loans | 2.9 |  | 2.7 |  |  |
Indirect consumer | 30 to 59 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 6.5 |  | 9.3 |  |  |
Indirect consumer | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 2.1 |  | 2.4 |  |  |
Indirect consumer | Equal to or Greater than 90 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.5 |  | 0.6 |  |  |
Indirect consumer | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 752.1 |  | 814.7 |  |  |
Direct and advance lines |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Allowance for credit losses |  |  |  | (5.0) | (4.6) | (4.6)
Financing Receivable, Loan in Process | 1.2 |  | 1.2 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 144.0 |  | 152.9 |  |  |
Nonaccrual loans | 0.2 |  | 0.3 |  |  |
Direct and advance lines | 30 to 59 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.6 |  | 0.8 |  |  |
Direct and advance lines | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.5 |  | 0.3 |  |  |
Direct and advance lines | Equal to or Greater than 90 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.1 |  | 0.1 |  |  |
Direct and advance lines | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 142.6 |  | 151.4 |  |  |
Credit card |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Allowance for credit losses |  |  |  | (2.2) | (2.3) | (2.2)
Financing Receivable, Loan in Process | 1.5 |  | 1.8 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 72.1 |  | 75.9 |  |  |
Nonaccrual loans | 0.0 |  | 0.0 |  |  |
Credit card | 30 to 59 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.7 |  | 0.8 |  |  |
Credit card | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.4 |  | 0.4 |  |  |
Credit card | Equal to or Greater than 90 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.4 |  | 0.6 |  |  |
Credit card | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 70.6 |  | 74.1 |  |  |
Total consumer loans |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Allowance for credit losses | (13.7) | (21.8) | (23.3) | (20.3) | (20.4) | (21.1)
Financing Receivable, Loan in Process | 11.8 |  | 15.3 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 980.2 |  | 1,058.5 |  |  |
Nonaccrual loans | 3.1 |  | 3.0 |  |  |
Total consumer loans | 30 to 59 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 7.8 |  | 10.9 |  |  |
Total consumer loans | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 3.0 |  | 3.1 |  |  |
Total consumer loans | Equal to or Greater than 90 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 1.0 |  | 1.3 |  |  |
Total consumer loans | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 965.3 |  | 1,040.2 |  |  |
Commercial |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Allowance for credit losses | (49.5) | (54.4) | (54.9) | (59.9) | (58.2) | (31.6)
Financing Receivable, Loan in Process | 8.0 |  | 10.9 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 3,002.7 |  | 2,882.6 |  |  |
Nonaccrual loans | 12.1 |  | 10.2 |  |  |
Commercial | 30 to 59 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 4.6 |  | 7.1 |  |  |
Commercial | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 2.3 |  | 1.7 |  |  |
Commercial | Equal to or Greater than 90 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 1.1 |  | 2.1 |  |  |
Commercial | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 2,982.6 |  | 2,861.5 |  |  |
Agricultural |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Allowance for credit losses | (2.7) | $ (2.4) | (3.2) | $ (6.5) | $ 1.0 | $ (0.3)
Financing Receivable, Loan in Process | 1.8 |  | 3.1 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 688.0 |  | 708.3 |  |  |
Nonaccrual loans | 2.8 |  | 8.7 |  |  |
Agricultural | 30 to 59 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 1.3 |  | 0.8 |  |  |
Agricultural | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.1 |  | 2.2 |  |  |
Agricultural | Equal to or Greater than 90 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.4 |  | 0.1 |  |  |
Agricultural | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 683.4 |  | 696.5 |  |  |
Other, including overdrafts |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.0 |  | 0.0 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 1.7 |  | 9.2 |  |  |
Nonaccrual loans | 0.0 |  | 0.0 |  |  |
Other, including overdrafts | 30 to 59 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.0 |  | 0.0 |  |  |
Other, including overdrafts | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.0 |  | 0.0 |  |  |
Other, including overdrafts | Equal to or Greater than 90 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.0 |  | 0.0 |  |  |
Other, including overdrafts | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 1.7 |  | 9.2 |  |  |
Loans And Leases Held For Investment [Member] |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 56.2 |  | 68.8 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 18,277.0 |  | 18,114.8 |  |  |
Nonaccrual loans | 86.1 |  | 59.2 |  |  |
Loans And Leases Held For Investment [Member] | 30 to 59 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 35.0 |  | 45.8 |  |  |
Loans And Leases Held For Investment [Member] | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 14.5 |  | 16.6 |  |  |
Loans And Leases Held For Investment [Member] | Equal to or Greater than 90 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 6.7 |  | 6.4 |  |  |
Loans And Leases Held For Investment [Member] | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | $ 18,134.7 |  | $ 17,986.8 |  |  |

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