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Loans and Loans Held for Sale - Additional Information (Details) $ in Thousands |  |  | 3 Months Ended | 6 Months Ended | 9 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2020 USD ($) loan | Dec. 31, 2019 USD ($) | Sep. 30, 2020 USD ($) loan | Jun. 30, 2020 USD ($) | Mar. 31, 2020 USD ($) | Sep. 30, 2019 USD ($) loan | Jun. 30, 2020 USD ($) loan | Sep. 30, 2020 USD ($) loan | Sep. 30, 2019 USD ($) loan | Dec. 31, 2019 USD ($) | Jan. 01, 2020 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Unearned income | $ 18,600 | $ 4,600 | $ 18,600 |  |  |  |  | $ 18,600 |  | $ 4,600 |
Purchase accounting fair value adjustments | 9,300 | 12,300 |  |  |  |  |  |  |  |  |
Total loans | 7,273,870 | 7,074,928 | 7,273,870 |  |  |  |  | 7,273,870 |  | 7,074,928 | $ 7,074,928
Loans held for investment, outstanding balance | $ 7,394,868 | $ 7,137,152 | 7,394,868 |  |  |  |  | $ 7,394,868 |  | $ 7,137,152 |
Threshold period of satisfactory performance for troubled debt restructuring to be restored to accruing status |  |  |  |  |  |  |  | 6 months |  |  |
Restructured loans |  |  | $ 4,562 |  |  | $ 25,800 |  | $ 11,586 | $ 36,712 |  |
Number of troubled debt restructuring loans returned to accruing status | loan |  |  | 2 |  |  | 3 |  | 3 | 5 |  |
Amount of trouble debt restructuring loans returned to accruing status |  |  | $ 100 |  |  | $ 200 |  | $ 22,700 | $ 200 |  |
Number of commitments to lend additional funds on TDRs | loan | 21 |  | 21 |  |  |  |  | 21 |  |  |
Commitments to lend additional funds on TDRs | $ 1,700 |  | $ 1,700 |  |  |  |  | $ 1,700 |  |  |
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 |  | 6 | 0 |  |
Amount of defaulted TDRs that were restructured within the last twelve months prior to defaulting |  |  |  |  |  |  |  | $ 18,100 |  |  |
CARES Act, Paycheck Protection Program Loans |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Number of modified loans | loan |  |  |  |  |  |  | 2,360 | 144 |  |  |
Modified loans |  |  |  |  |  |  | $ 1,400,000 | $ 350,000 |  |  |
Commercial |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Percentage of commercial loans in total portfolio loans | 78.60% | 77.20% | 78.60% |  |  |  |  | 78.60% |  | 77.20% |
Loans held for investment, outstanding balance | $ 5,810,034 | $ 5,512,796 | $ 5,810,034 |  |  |  |  | $ 5,810,034 |  | $ 5,512,796 |
Commercial real estate | Commercial |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Loans held for investment, outstanding balance | 2,818,227 | 3,059,592 | 2,818,227 |  |  |  |  | 2,818,227 |  | 3,059,592 | 3,416,518
Restructured loans |  |  | 171 |  |  | $ 23,784 |  | 2,381 | $ 29,747 |  |
Commercial and industrial | Commercial |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Loans held for investment, outstanding balance | 1,818,649 | 1,480,529 | 1,818,649 |  |  |  |  | 1,818,649 |  | 1,480,529 | $ 1,720,833
Restructured loans |  |  | 4,022 |  |  | $ 1,527 |  | 5,381 | $ 5,860 |  |
CRE and commercial construction | Commercial |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Loans held for investment, outstanding balance | $ 3,800,000 | $ 3,800,000 | $ 3,800,000 |  |  |  |  | $ 3,800,000 |  | $ 3,800,000 |
Combined percentage of commercial real estate and commercial construction in total commercial loans | 64.80% | 68.80% | 64.80% |  |  |  |  | 64.80% |  | 68.80% |
Combined percentage of commercial real estate and commercial construction in total portfolio loans | 50.90% | 53.10% | 50.90% |  |  |  |  | 50.90% |  | 53.10% |
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) | 15.00% | 11.00% | 15.00% |  |  |  |  | 15.00% |  | 11.00% |
Out of market exposure of combined portfolio (percent) | 6.00% | 5.40% | 6.00% |  |  |  |  | 6.00% |  | 5.40% |
Percentage of total loans out-of-state excluding contiguous states | 3.10% | 2.90% | 3.10% |  |  |  |  | 3.10% |  | 2.90% |
CARES Act, Paycheck Protection Program Loans |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total loans | $ 550,100 |  | $ 550,100 |  |  |  |  | $ 550,100 |  |  |
Restructured loans |  |  |  |  |  |  |  | 350,000 |  |  |
CARES Act, Paycheck Protection Program Loans | Commercial |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total loans | 550,100 |  | 550,100 |  |  |  |  | 550,100 |  |  |
Nonperforming TDRs | Customer fraud |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Restructured loans |  |  |  | $ 10,900 |  |  |  | 10,900 |  |  |
Nonperforming TDRs | Commercial real estate | Commercial |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Restructured loans |  |  |  |  | $ 21,300 |  |  | 21,300 |  |  |
Amount charged down |  |  | 10,000 |  |  |  |  |  |  |  |
Outstanding balance after charge down | $ 11,300 |  | 11,300 |  |  |  |  | 11,300 |  |  |
Nonperforming TDRs | Commercial and industrial | Commercial |  |  |  |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Restructured loans |  |  | $ 4,900 |  |  |  |  | $ 4,900 |  |  |

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