Loans (Narrative) (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) Item	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Nov. 30, 2015	Dec. 31, 2012 USD ($)
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	$ 7,700
Minimum number of days to be considered past due	90 days
Troubled debt restructurings	$ 536	8,339	$ 11,215
Number of loans restructured during the last year in excess of 90 days past due | Item	1
Previously restructured loans past due	$ 259
Financing Receivable, Modifications, Pre-Modification Recorded Investment	$ 709	$ 8,916	14,833
Number of days past due for payment of interest on loans, minimum	120 days
Texas Leading Index economic condition indicator		129.3		124.1
Maximum origination value of loans which were subject to further refinement of valuation methodology	$ 1,000
General valuation allowance allocations for loan agreement monitoring exceptions	$ 135,859	$ 99,542	92,438		$ 104,453
Sensitivity stress test price shock	75.00%
Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
General valuation allowance allocations for loan agreement monitoring exceptions	$ 24,313	$ 27,163	22,590		29,346
Commercial And Industrial Energy Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of loans related to a single industry	15.30%	16.10%
Troubled debt restructurings	$ 0	$ 0	531
Financing Receivable, Modifications, Pre-Modification Recorded Investment	0	0	528
General valuation allowance allocations for loan agreement monitoring exceptions	54,696	14,919	6,090		7,579
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings	536	8,300
Additional interest that would have been recognized on non-accrual loans, net of tax	1,600	1,500	2,200
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Troubled debt restructurings	536	5,391	4,937
Financing Receivables, Impaired, Troubled Debt Restructuring, Write-down	88	627
Financing Receivable, Modifications, Pre-Modification Recorded Investment	709	5,795	6,334
General valuation allowance allocations for loan agreement monitoring exceptions	$ 42,993	44,273	46,700		$ 46,585
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]
Troubled debt restructurings	$ 0	2,948	5,747
Financing Receivable, Modifications, Pre-Modification Recorded Investment	0	3,121	$ 7,964
Previously Restructured Loans Moved to OREO		500
Amount charged off related to loans restructured		0
Credit And Collateral Exceptions [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
General valuation allowance allocations for loan agreement monitoring exceptions	0	2,200
Commercial Borrower [Member] | Commercial Real Estate Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Modifications, Pre-Modification Recorded Investment		2,900
Commercial Borrower [Member] | Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Modifications, Pre-Modification Recorded Investment	$ 277	$ 2,700