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Loans Held for Investment - Schedule of Loans by Segment (Details) - USD ($) $ in Millions | Jun. 30, 2024 | Mar. 31, 2024 | Dec. 31, 2023 | Jun. 30, 2023 | Mar. 31, 2023 | Dec. 31, 2022
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Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Deferred loan fees and costs | $ (12.3) |  | $ (12.5) |  |  |
Loans held for investment, net of deferred fees and costs | 18,235.0 |  | 18,279.6 |  |  |
Allowance for credit losses | (232.8) | $ (227.7) | (227.7) | $ (224.6) | $ (226.1) | $ (220.1)
Net loans held for investment | 18,002.2 |  | 18,051.9 |  |  |
Financing Receivable, Loan in Process | 1.2 |  |  |  |  |
Commercial real estate |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 6.5 |  | 18.8 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 9,054.5 |  | 8,869.2 |  |  |
Nonaccrual loans | 37.7 |  | 28.2 |  |  |
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 |  | 12.7 |  |  |
Commercial real estate | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 2.3 |  | 6.1 |  |  |
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 | 0.1 |  | 0.0 |  |  |
Commercial real estate | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 9,010.3 |  | 8,822.2 |  |  |
Total construction loans |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 2.3 |  | 3.5 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 1,519.9 |  | 1,826.5 |  |  |
Nonaccrual loans | 9.4 |  | 17.2 |  |  |
Total construction loans | 30 to 59 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.3 |  | 3.1 |  |  |
Total construction loans | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 2.0 |  | 0.4 |  |  |
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 | 0.0 |  | 0.0 |  |  |
Total construction loans | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 1,508.2 |  | 1,805.8 |  |  |
Residential |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 8.7 |  | 15.6 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 2,246.4 |  | 2,244.3 |  |  |
Nonaccrual loans | 13.3 |  | 10.7 |  |  |
Residential | 30 to 59 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 3.3 |  | 11.9 |  |  |
Residential | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 4.3 |  | 3.1 |  |  |
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 | 1.1 |  | 0.6 |  |  |
Residential | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 2,224.4 |  | 2,218.0 |  |  |
Agricultural |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 4.1 |  | 1.8 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 723.5 |  | 716.8 |  |  |
Nonaccrual loans | 4.8 |  | 5.4 |  |  |
Agricultural | 30 to 59 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 1.3 |  | 1.8 |  |  |
Agricultural | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 2.8 |  | 0.0 |  |  |
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 | 0.0 |  | 0.0 |  |  |
Agricultural | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 714.6 |  | 709.6 |  |  |
Total real estate |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Allowance for credit losses | (139.7) | (152.9) | (160.1) | (158.7) | (147.5) | (138.7)
Financing Receivable, Loan in Process | 21.6 |  | 39.7 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 13,544.3 |  | 13,656.8 |  |  |
Nonaccrual loans | 65.2 |  | 61.5 |  |  |
Total real estate | 30 to 59 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 9.0 |  | 29.5 |  |  |
Total real estate | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 11.4 |  | 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 | 1.2 |  | 0.6 |  |  |
Total real estate | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 13,457.5 |  | 13,555.6 |  |  |
Indirect consumer |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 9.0 |  | 10.6 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 733.7 |  | 740.9 |  |  |
Nonaccrual loans | 3.2 |  | 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 |  | 8.0 |  |  |
Indirect consumer | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 2.5 |  | 2.2 |  |  |
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 | 0.0 |  | 0.4 |  |  |
Indirect consumer | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 721.5 |  | 727.6 |  |  |
Direct and advance lines |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.8 |  | 1.1 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 139.0 |  | 141.6 |  |  |
Nonaccrual loans | 0.5 |  | 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 |  | 0.9 |  |  |
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 | 0.2 |  | 0.2 |  |  |
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 | 0.0 |  | 0.0 |  |  |
Direct and advance lines | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 137.7 |  | 140.2 |  |  |
Credit card |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 1.7 |  | 1.8 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 76.1 |  | 76.5 |  |  |
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 |  | 0.7 |  |  |
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 |  | 0.5 |  |  |
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 | 0.6 |  | 0.6 |  |  |
Credit card | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 74.4 |  | 74.7 |  |  |
Total consumer loans |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Allowance for credit losses | (18.5) | (12.6) | (13.0) | (13.7) | (21.8) | (23.3)
Financing Receivable, Loan in Process | 11.5 |  | 13.5 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 948.8 |  | 959.0 |  |  |
Nonaccrual loans | 3.7 |  | 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 |  | 9.6 |  |  |
Total consumer loans | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 3.1 |  | 2.9 |  |  |
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 | 0.6 |  | 1.0 |  |  |
Total consumer loans | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 933.6 |  | 942.5 |  |  |
Commercial |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Allowance for credit losses | (69.2) | (58.0) | (50.2) | (49.5) | (54.4) | (54.9)
Financing Receivable, Loan in Process | 13.9 |  | 15.9 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 3,052.9 |  | 2,906.8 |  |  |
Nonaccrual loans | 72.6 |  | 11.5 |  |  |
Commercial | 30 to 59 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 11.3 |  | 14.5 |  |  |
Commercial | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 1.8 |  | 1.1 |  |  |
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 | 0.8 |  | 0.3 |  |  |
Commercial | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 2,966.4 |  | 2,879.4 |  |  |
Agricultural |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Allowance for credit losses | (5.4) | $ (4.2) | (4.4) | $ (2.7) | $ (2.4) | $ (3.2)
Financing Receivable, Loan in Process | 2.0 |  | 3.1 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 698.2 |  | 769.4 |  |  |
Nonaccrual loans | 24.1 |  | 30.4 |  |  |
Agricultural | 30 to 59 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 1.7 |  | 0.1 |  |  |
Agricultural | 60 to 89 Days Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 0.3 |  | 0.0 |  |  |
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 | 0.0 |  | 3.0 |  |  |
Agricultural | Financial Asset, Not Past Due |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 672.1 |  | 735.9 |  |  |
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 | 3.1 |  | 0.1 |  |  |
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 | 3.1 |  | 0.1 |  |  |
Loans And Leases Held For Investment [Member] |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Financing Receivable, Loan in Process | 49.0 |  | 72.2 |  |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 18,247.3 |  | 18,292.1 |  |  |
Nonaccrual loans | 165.6 |  | 106.4 |  |  |
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 | 29.8 |  | 53.7 |  |  |
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 | 16.6 |  | 13.6 |  |  |
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 | 2.6 |  | 4.9 |  |  |
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 | $ 18,032.7 |  | $ 18,113.5 |  |  |

X
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