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Loans - Additional Information (Detail) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2021 | Jun. 30, 2020 | Jun. 30, 2021 | Jun. 30, 2020 | Mar. 31, 2021 | Dec. 31, 2020 | Mar. 31, 2020 | Dec. 31, 2019
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% |  | 10.00% |  |  |  |  |
Loans and Leases Receivable, Related Parties | $ 289,700 |  | $ 289,700 |  |  | $ 353,100 |  |
Texas Leading Index economic condition indicator | 131.5 |  | 131.5 |  |  | 118.4 |  |
Financing Receivable, Allowance for Credit Loss | $ 255,288 |  | $ 255,288 |  |  | $ 263,177 |  |
Non Accrual Loans [Member] |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |
Additional interest that would have been recognized on non-accrual loans, net of tax | $ 427 | $ 570 | $ 881 | $ 1,600 |  |  |  |
Commercial and Industrial Total Energy Loans [Member] |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |
Percentage of total energy loans to total loans | 6.20% |  | 6.20% |  |  | 7.10% |  |
Percentage of total energy loans to total loans excluding PPP Loans | 7.00% |  | 7.00% |  |  |  |  |
Financing Receivable, Allowance for Credit Loss | $ 28,010 | $ 40,817 | $ 28,010 | $ 40,817 | $ 33,472 | $ 39,553 | $ 103,201 | $ 37,382
Paycheck Protection Program [Member] |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |
Financing Receivable, Allowance for Credit Loss | 0 |  | 0 |  |  | 0 |  |
Commitments to Extend Credit [Member] | Commercial and industrial, Energy [Member] |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |
Unfunded balances | 930,800 |  | 930,800 |  |  |  |  |
Standby Letters of Credit [Member] | Commercial and industrial, Energy [Member] |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |
Unfunded balances | 60,300 |  | 60,300 |  |  |  |  |
Loans Receivable [Member] |  |  |  |  |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |  |  |  |  |
Interest Receivable | $ 44,600 |  | $ 44,600 |  |  | $ 48,700 |  |

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