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Loans - Additional Information (Detail)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2016
USD ($)
Jun. 30, 2015
USD ($)
Jun. 30, 2016
USD ($)
Jun. 30, 2015
USD ($)
May 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%      
Percentage of total energy loans to total loans 13.00%   13.00%     15.30%
Troubled debt restructurings     $ 22,980 $ 584    
Amount charged off of previously restructured loans     9,500      
Proceeds from sale of loans     30,470 0    
Texas Leading Index economic condition indicator         122.9 123.0
Commercial and industrial, Energy [Member]            
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Troubled debt restructurings     20,795 0    
Non-Accrual Loans [Member]            
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Troubled debt restructurings     21,000      
Additional interest that would have been recognized on non-accrual loans, net of tax $ 936 $ 364 1,800 $ 760    
Commitments to Extend Credit [Member] | Commercial and industrial, Energy [Member]            
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Unfunded balances 1,100,000   1,100,000      
Standby Letters of Credit [Member] | Commercial and industrial, Energy [Member]            
Accounts, Notes, Loans and Financing Receivable [Line Items]            
Unfunded balances $ 60,800   $ 60,800