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Loans (Narrative) (Detail) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 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% | 10.00% |
Commercial and industrial loans related to overdraft for correspondent bank customer | $ 7,300,000 | $ 6,300,000 |
Minimum number of days to be considered past due | 90 days |  |
Restructured loans on non-accrual status at period end | $ 58,484,000 | 44,863,000 | $ 536,000
Interest income recognized subsequent to classification as impaired | $ 0 | $ 0 | 0
Number of days past due for payment of interest on loans, minimum | 120 days |  |
Texas Leading Index economic condition indicator | 128.7 | 123.1 |
Maximum origination value of loans which were subject to further refinement of valuation methodology | $ 1,000,000 |  |
Sensitivity stress test price shock | 75.00% |  |
Commercial And Industrial Energy Financing Receivable [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Percentage of loans related to a single industry | 11.40% | 11.60% |
Restructured loans on non-accrual status at period end | $ 54,330,000 | $ 43,841,000 | 0
Non Accrual Loans [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Restructured loans on non-accrual status at period end | 53,622,000 | 44,863,000 | 536,000
Additional interest that would have been recognized on non-accrual loans, net of tax | 3,700,000 | 3,100,000 | 1,600,000
Commercial Portfolio Segment [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Restructured loans on non-accrual status at period end | $ 3,766,000 | 1,022,000 | 536,000
Grade 12 [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
General percentage of valuation allowance in excess of principal balance, maximum | 30.00% |  |
Grade 13 [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
General percentage of valuation allowance in excess of principal balance, minimum | 30.00% |  |
Commercial and Industrial, Other Commercial [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Number of days past due for payment of interest on loans, minimum | 180 days |  |
Commercial Real Estate Buildings Land And Other Financing Receivable [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Restructured loans on non-accrual status at period end | $ 0 | $ 0 | $ 0
Commitments to Extend Credit [Member] | Commercial And Industrial Energy Financing Receivable [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Unfunded balances | 1,100,000,000 |  |
Standby Letters of Credit [Member] | Commercial And Industrial Energy Financing Receivable [Member] |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Unfunded balances | $ 46,700,000 |  |

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