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Loans - Schedule of Loans by Segment (Details) - USD ($) $ in Millions | Sep. 30, 2020 | Jun. 30, 2020 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 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 | $ (34.4) |  | (5.5) |  |  |
Loans held for investment, net of deferred fees and costs | 10,152.2 |  | 8,930.7 |  |  |
Allowance for credit losses | (145.5) | $ (146.1) | (73.0) | $ (75.0) | $ (74.2) | $ (73.0)
Net loans held for investment | 10,006.7 |  | 8,857.7 |  |  |
Commercial real estate |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 3,690.9 |  | 3,487.8 |  |  |
Allowance for credit losses | (55.1) | (51.2) | (19.5) |  |  |
Land acquisition & development |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 274.8 |  | 302.1 |  |  |
Allowance for credit losses | (1.2) | (1.5) | (1.9) |  |  |
Residential |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 227.9 |  | 244.1 |  |  |
Allowance for credit losses | (1.6) | (1.3) | (1.5) |  |  |
Commercial |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 530.8 |  | 431.5 |  |  |
Allowance for credit losses | (7.1) | (6.3) | (2.7) |  |  |
Total construction loans |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 1,033.5 |  | 977.7 |  |  |
Residential |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 1,311.2 |  | 1,246.1 |  |  |
Allowance for credit losses | (10.7) | (11.8) | (2.8) |  |  |
Agricultural |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 227.7 |  | 226.6 |  |  |
Allowance for credit losses | (2.9) | (3.1) | (0.5) |  |  |
Total real estate loans |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 6,263.3 |  | 5,938.2 |  |  |
Allowance for credit losses | (78.6) | (75.2) | (28.9) |  |  |
Indirect consumer |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 812.8 |  | 784.6 |  |  |
Allowance for credit losses | (17.0) | (16.4) | (4.5) |  |  |
Direct and advance lines |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 162.1 |  | 179.0 |  |  |
Allowance for credit losses | (4.8) | (5.1) | (2.9) |  |  |
Credit card |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 69.9 |  | 81.6 |  |  |
Allowance for credit losses | (2.2) | (2.0) | (2.5) |  |  |
Total consumer loans |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 1,044.8 |  | 1,045.2 |  |  |
Allowance for credit losses | (24.0) | (23.5) | (9.9) |  |  |
Commercial |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 2,599.6 |  | 1,673.7 |  |  |
Allowance for credit losses | (42.0) | (46.5) | (32.6) |  |  |
Agricultural |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 274.7 |  | 279.1 |  |  |
Allowance for credit losses | (0.9) | $ (0.9) | (1.6) |  |  |
Other, including overdrafts |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | 4.2 |  | 0.0 |  |  |
Loans And Leases Held For Investment [Member] |  |  |  |  |  |
Loans and Leases Receivable Recorded Investment, Past Due [Line Items] |  |  |  |  |  |
Loans held for investment | $ 10,186.6 |  | $ 8,936.2 |  |  |

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