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Loans - Additional Information (Detail) $ in Thousands | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2016 USD ($) Item | Mar. 31, 2015 USD ($) | Feb. 29, 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% |  |  |
Maximum percentage loan related to single industry | 10.00% |  |  |
Percentage of total energy loans to total loans | 14.40% |  |  | 15.30%
Troubled debt restructurings | $ 61,347 | $ 613 |  |
Number of loans restructured during the last year in excess of 90 days past due | Item | 1 |  |  |
Previously restructured loans past due | $ 17 |  |  |
Minimum number of days to be considered past due | 90 days |  |  |
Texas Leading Index economic condition indicator |  |  | 120.6 | 123.0
Commercial and industrial, Energy [Member] |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Troubled debt restructurings | $ 61,095 | 0 |  |
Non-Accrual Loans [Member] |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Troubled debt restructurings | 61,300 |  |  |
Additional interest that would have been recognized on non-accrual loans, net of tax | 844 | $ 397 |  |
Commitments to Extend Credit [Member] | Commercial and industrial, Energy [Member] |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Unfunded balances | 1,300,000 |  |  |
Standby Letters of Credit [Member] | Commercial and industrial, Energy [Member] |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Unfunded balances | $ 61,100 |  |  |

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