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Loans - Additional Information (Detail) $ in Thousands | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($) | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($) | Aug. 31, 2016 | Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Concentration of loans in a single industry in excess of ten percent | 0.00% |  | 0.00% |  |  |
Maximum percentage loan related to single industry |  |  | 10.00% |  |  |
Percentage of total energy loans to total loans | 12.00% |  | 12.00% |  |  | 15.30%
Troubled debt restructurings |  |  | $ 34,099 | $ 554 |  |
Amount charged off of previously restructured loans |  |  | 9,500 |  |  |
Proceeds from sale of loans |  |  | 30,470 | 0 |  |
Texas Leading Index economic condition indicator |  |  |  |  | 122.7 | 123.0
Financing Receivables, Impaired, Troubled Debt Restructuring, Write-down |  |  | 4,000 |  |  |
Commercial and industrial, Energy [Member] |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Troubled debt restructurings |  |  | 31,918 | 0 |  |
Non-Accrual Loans [Member] |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Troubled debt restructurings |  |  | 32,200 |  |  |
Additional interest that would have been recognized on non-accrual loans, net of tax | $ 647 | $ 352 | 2,400 | $ 1,100 |  |
Commitments to Extend Credit [Member] | Commercial and industrial, Energy [Member] |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Unfunded balances | 1,100,000 |  | 1,100,000 |  |  |
Standby Letters of Credit [Member] | Commercial and industrial, Energy [Member] |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |
Unfunded balances | $ 58,700 |  | $ 58,700 |  |  |

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