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Loans - Additional Information (Detail)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2015
USD ($)
Item
Sep. 30, 2014
USD ($)
Sep. 30, 2015
USD ($)
Item
Sep. 30, 2014
USD ($)
Aug. 31, 2015
Jun. 30, 2015
USD ($)
Dec. 31, 2014
USD ($)
Jun. 30, 2014
USD ($)
Dec. 31, 2013
USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]                  
Number Of Loans Restructured During The Last Year In Excess of 90 Days Past Due | Item 1   1            
Maximum loan-to-value percentage on home equity loans     80.00%            
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%            
Minimum number of days to be considered past due     90 days            
Troubled debt restructurings     $ 554 $ 6,524          
Previously restructured loans past due     268            
Financing receivable, modifications, pre-modification recorded investment     $ 709 6,874          
Number of days past due for payment of interest on loans, minimum     120 days            
Texas Leading Index economic condition indicator         123.7   129.3    
Maximum origination value of loans which were subject to further refinement of valuation methodology     $ 1,000            
Loans and Leases Receivable, Allowance $ 110,373 $ 98,312 $ 110,373 98,312   $ 106,607 $ 99,542 $ 98,286 $ 92,438
Commercial Real Estate Portfolio Segment [Member]                  
Accounts, Notes, Loans and Financing Receivable [Line Items]                  
Percentage of owner-occupied commercial real estate loans 52.00%   52.00%            
Previously Restructured Loans Moved to OREO       500          
Charge-off of previously restructures loans       0          
Loans and Leases Receivable, Allowance $ 25,883 26,780 $ 25,883 26,780   24,935 27,163 22,046 22,590
Commercial and industrial, Energy [Member]                  
Accounts, Notes, Loans and Financing Receivable [Line Items]                  
Percentage of loans related to a single industry     15.70%            
Troubled debt restructurings     $ 0 0          
Financing receivable, modifications, pre-modification recorded investment     0 0          
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 352 414 1,112 1,127          
Commercial Portfolio Segment [Member]                  
Accounts, Notes, Loans and Financing Receivable [Line Items]                  
Financing receivables, impaired, troubled debt restructuring, write-down     88            
Financing receivable, modifications, pre-modification recorded investment     $ 286            
Charge-off of previously restructures loans       627          
Number of days past due for payment of interest on loans, minimum     180 days            
Loans and Leases Receivable, Allowance 70,803 $ 58,636 $ 70,803 $ 58,636   $ 68,725 59,192 $ 55,672 $ 52,790
Grade 12 [Member]                  
Accounts, Notes, Loans and Financing Receivable [Line Items]                  
Number of days past due for payment of interest on loans, minimum     120 days            
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%            
Credit and Collateral Exceptions [Member]                  
Accounts, Notes, Loans and Financing Receivable [Line Items]                  
Loans and Leases Receivable, Allowance $ 0   $ 0       2,164    
Credit and Collateral Exceptions [Member] | Commercial Real Estate Portfolio Segment [Member]                  
Accounts, Notes, Loans and Financing Receivable [Line Items]                  
Loans and Leases Receivable, Allowance             681    
Credit and Collateral Exceptions [Member] | Commercial Portfolio Segment [Member]                  
Accounts, Notes, Loans and Financing Receivable [Line Items]                  
Loans and Leases Receivable, Allowance             $ 1,483