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Loans - Schedule of Loans by Segment (Details) - USD ($) $ in Millions | Jun. 30, 2020 | Mar. 31, 2020 | Dec. 31, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | --- | --- | --- | --- | ---
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment |  |  | $ 8,936.2 |  |  |
Deferred loan fees and costs | $ (41.3) |  | (5.5) |  |  |
Loans held for investment, net of deferred fees and costs | 10,032.5 |  | 8,930.7 |  |  |
Allowance for credit losses | (146.1) | $ (129.1) | (73.0) | $ (74.2) | $ (72.4) | $ (73.0)
Net loans held for investment | 9,886.4 |  | 8,857.7 |  |  |
Commercial real estate |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 3,593.8 |  | 3,487.8 |  |  |
Allowance for credit losses | (51.2) | (39.5) | (19.5) |  |  |
Land acquisition & development |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 285.3 |  | 302.1 |  |  |
Allowance for credit losses | (1.5) | (1.4) | (1.9) |  |  |
Residential |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 246.2 |  | 244.1 |  |  |
Allowance for credit losses | (1.3) | (1.1) | (1.5) |  |  |
Commercial |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 459.8 |  | 431.5 |  |  |
Allowance for credit losses | (6.3) | (6.1) | (2.7) |  |  |
Total construction loans |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 991.3 |  | 977.7 |  |  |
Residential |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 1,287.6 |  | 1,246.1 |  |  |
Allowance for credit losses | (11.8) | (14.3) | (2.8) |  |  |
Agricultural |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 224.2 |  | 226.6 |  |  |
Allowance for credit losses | (3.1) | (2.6) | (0.5) |  |  |
Total real estate loans |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 6,096.9 |  | 5,938.2 |  |  |
Allowance for credit losses | (75.2) | (65.0) | (28.9) |  |  |
Indirect consumer |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 801.9 |  | 784.6 |  |  |
Allowance for credit losses | (16.4) | (16.7) | (4.5) |  |  |
Direct and advance lines |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 169.3 |  | 179.0 |  |  |
Allowance for credit losses | (5.1) | (5.5) | (2.9) |  |  |
Credit card |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 70.6 |  | 81.6 |  |  |
Allowance for credit losses | (2.0) | (2.6) | (2.5) |  |  |
Total consumer loans |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 1,041.8 |  | 1,045.2 |  |  |
Allowance for credit losses | (23.5) | (24.8) | (9.9) |  |  |
Commercial |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 2,648.6 |  | 1,673.7 |  |  |
Allowance for credit losses | (46.5) | (38.2) | (32.6) |  |  |
Agricultural |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 282.8 |  | 279.1 |  |  |
Allowance for credit losses | (0.9) | $ (1.1) | (1.6) |  |  |
Other, including overdrafts |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 3.7 |  | 0.0 |  |  |
Loans And Leases Held For Investment [Member] |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | $ 10,073.8 |  | $ 8,936.2 |  |  |

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