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Loans and Loans Held for Sale - Additional Information (Details) | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | ---
Sep. 30, 2018 USD ($) loan | Mar. 31, 2018 USD ($) | Sep. 30, 2017 USD ($) loan | Sep. 30, 2018 USD ($) loan | Sep. 30, 2017 USD ($) loan | Dec. 31, 2017 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Loans, net of unearned income | $ 5,400,000 |  |  | $ 5,400,000 |  | $ 5,200,000
Threshold period of satisfactory performance for troubled debt restructuring to be restored to accruing status |  |  |  | 6 months |  |
Number of troubled debt restructuring loans returned to accruing status | loan | 0 |  | 0 | 0 | 1 |
Amount of trouble debt restructuring loans returned to accruing status | $ 0 |  | $ 0 | $ 0 | $ 2,000,000 |
Number of commitments to lend additional funds on TDRs | loan | 12 |  |  | 12 |  |
Commitments to lend additional funds on TDRs | $ 6,500,000 |  |  | $ 6,500,000 |  |
Minimum period of loan payment defaults following restructure for TDRs to be in default |  |  |  | 90 days |  |
Number of defaulted TDRs that were restructured within the last twelve months prior to defaulting | loan | 0 |  | 0 | 0 | 0 |
Increase in other real estate |  |  |  | $ 2,600,000 |  |
Land lots |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Increase in other real estate |  | $ 2,500,000 |  |  |  |
Commercial |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Percentage of commercial loans in total portfolio loans | 79.00% |  |  | 79.00% |  | 78.00%
Commercial loans | $ 4,561,526,000 |  |  | $ 4,561,526,000 |  | $ 4,503,594,000
Commercial real estate and commercial construction | Commercial |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Commercial loans | $ 3,100,000,000 |  |  | $ 3,100,000,000 |  | $ 3,100,000,000
Combined percentage of commercial real estate and commercial construction in total commercial loans | 68.00% |  |  | 68.00% |  | 68.00%
Combined percentage of commercial real estate and commercial construction in total portfolio loans | 54.00% |  |  | 54.00% |  | 53.00%
Concentration risk percentage of commercial real estate and commercial construction |  |  |  | 0.00% |  | 0.00%
Maximum concentration of commercial real estate and commercial construction portfolio in loans (in excess of) | 14.00% |  |  | 14.00% |  | 14.00%
Out of market exposure of combined portfolio (percent) | 5.50% |  |  | 5.50% |  | 5.20%
Percentage of total loans out-of-state excluding contiguous states | 2.90% |  |  | 2.90% |  | 2.80%

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