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Loans Held for Investment - Schedule of Loans by Segment (Details) - USD ($) $ in Millions | Mar. 31, 2024 | Dec. 31, 2023 | Mar. 31, 2023 | Dec. 31, 2022
--- | --- | --- | --- | ---
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Deferred loan fees and costs | $ (12.5) | $ (12.5) |  |
Loans held for investment, net of deferred fees and costs | 18,202.8 | 18,279.6 |  |
Allowance for credit losses | (227.7) | (227.7) | $ (226.1) | $ (220.1)
Net loans held for investment | 17,975.1 | 18,051.9 |  |
Financing Receivable, Loan in Process | 19.1 |  |  |
Commercial real estate |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 1.7 | 18.8 |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 9,060.4 | 8,869.2 |  |
Nonaccrual loans | 37.0 | 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 | 1.5 | 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 | 0.2 | 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 | 0.0 | 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 | 9,021.7 | 8,822.2 |  |
Total construction loans |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 2.2 | 3.5 |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 1,609.2 | 1,826.5 |  |
Nonaccrual loans | 16.3 | 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 | 0.9 | 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 | 1.2 | 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 | 0.1 | 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 | 1,590.7 | 1,805.8 |  |
Residential |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 14.4 | 15.6 |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 2,258.4 | 2,244.3 |  |
Nonaccrual loans | 12.5 | 10.7 |  |
Residential | 30 to 59 Days Past Due |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 13.2 | 11.9 |  |
Residential | 60 to 89 Days Past Due |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 0.3 | 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 | 0.9 | 0.6 |  |
Residential | Financial Asset, Not Past Due |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 2,231.5 | 2,218.0 |  |
Agricultural |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 5.7 | 1.8 |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 719.7 | 716.8 |  |
Nonaccrual loans | 4.9 | 5.4 |  |
Agricultural | 30 to 59 Days Past Due |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 0.1 | 1.8 |  |
Agricultural | 60 to 89 Days Past Due |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 5.6 | 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 | 709.1 | 709.6 |  |
Total real estate |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Allowance for credit losses | (152.9) | (160.1) | (147.5) | (138.7)
Financing Receivable, Loan in Process | 24.0 | 39.7 |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 13,647.7 | 13,656.8 |  |
Nonaccrual loans | 70.7 | 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 | 15.7 | 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 | 7.3 | 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 | 1.0 | 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 | 13,553.0 | 13,555.6 |  |
Indirect consumer |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 8.3 | 10.6 |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 739.9 | 740.9 |  |
Nonaccrual loans | 3.4 | 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 | 6.1 | 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 | 1.9 | 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 | 0.3 | 0.4 |  |
Indirect consumer | Financial Asset, Not Past Due |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 728.2 | 727.6 |  |
Direct and advance lines |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 1.3 | 1.1 |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 136.7 | 141.6 |  |
Nonaccrual loans | 0.4 | 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 | 0.8 | 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 | 0.4 | 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 | 0.1 | 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 | 135.0 | 140.2 |  |
Credit card |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 1.4 | 1.8 |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 72.6 | 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 | 0.5 | 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 | 0.2 | 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 | 0.7 | 0.6 |  |
Credit card | Financial Asset, Not Past Due |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 71.2 | 74.7 |  |
Total consumer loans |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Allowance for credit losses | (12.6) | (13.0) | (21.8) | (23.3)
Financing Receivable, Loan in Process | 11.0 | 13.5 |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 949.2 | 959.0 |  |
Nonaccrual loans | 3.8 | 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 | 7.4 | 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 | 2.5 | 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 | 1.1 | 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 | 934.4 | 942.5 |  |
Commercial |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Allowance for credit losses | (58.0) | (50.2) | (54.4) | (54.9)
Financing Receivable, Loan in Process | 11.6 | 15.9 |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 2,922.2 | 2,906.8 |  |
Nonaccrual loans | 67.6 | 11.5 |  |
Commercial | 30 to 59 Days Past Due |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 10.3 | 14.5 |  |
Commercial | 60 to 89 Days Past Due |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 0.5 | 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 | 2,843.0 | 2,879.4 |  |
Agricultural |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Allowance for credit losses | (4.2) | (4.4) | $ (2.4) | $ (3.2)
Financing Receivable, Loan in Process | 19.2 | 3.1 |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 696.0 | 769.4 |  |
Nonaccrual loans | 29.9 | 30.4 |  |
Agricultural | 30 to 59 Days Past Due |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 19.1 | 0.1 |  |
Agricultural | 60 to 89 Days Past Due |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 0.0 | 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 | 0.1 | 3.0 |  |
Agricultural | Financial Asset, Not Past Due |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 646.9 | 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 | 0.2 | 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 | 0.2 | 0.1 |  |
Loans And Leases Held For Investment [Member] |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |
Financing Receivable, Loan in Process | 65.8 | 72.2 |  |
Financing Receivable, Excluding Accrued Interest, before Allowance for Credit Loss | 18,215.3 | 18,292.1 |  |
Nonaccrual loans | 172.0 | 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 | 52.5 | 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 | 10.3 | 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 | 3.0 | 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 | $ 17,977.5 | $ 18,113.5 |  |

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