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Loans - Additional Information (Detail)
$ in Thousands
 3 Months Ended
Mar. 31, 2025
USD ($)
Mar. 31, 2024
USD ($)
Dec. 31, 2024
USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Concentration of loans in a single industry in excess of ten percent 0.00%    
Maximum percentage loan related to single industry 10.00%    
U.S. dollar denominated loans and commitments to borrowers in Mexico $ 0   $ 0
Loans and Leases Receivable, Related Parties 310,800   295,800
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged 19,200,000   19,200,000
Federal Home Loan Bank Borrowings, Fair Value Disclosure 0   0
Loans, net of unearned discounts $ 20,903,920   $ 20,754,813
Texas Leading Index economic condition indicator 123.9   125.2
Combination: Payment Delay and Term Extention      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans, net of unearned discounts $ 0 $ 28,931  
Financing Receivables, 30 to 89 Days Past Due [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans, net of unearned discounts $ 126,752    
Automobile Loan [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Percentage of total loans 5.70%    
Commercial and Industrial Total Energy Loans [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Percentage of total loans 5.50%    
Loans, net of unearned discounts $ 1,153,137   $ 1,128,895
Commercial and Industrial Total Energy Loans [Member] | Financing Receivables, 30 to 89 Days Past Due [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans, net of unearned discounts 6,980    
Non Accrual Loans [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Financing Receivable, Excluding Accrued Interest, Impaired Interest Lost On Nonaccrual Loans 1,400 1,200  
Total consumer real estate loans [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans, net of unearned discounts 3,203,544   3,103,389
Total consumer real estate loans [Member] | Financing Receivables, 30 to 89 Days Past Due [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans, net of unearned discounts 18,731    
Commercial Portfolio Segment [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans, net of unearned discounts 6,163,093   6,109,532
Commercial Portfolio Segment [Member] | Additional Funding Agreement Terms      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans, net of unearned discounts 0    
Commercial Portfolio Segment [Member] | Combination: Payment Delay and Term Extention      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans, net of unearned discounts 0 $ 28,931  
Commercial Portfolio Segment [Member] | Financing Receivables, 30 to 89 Days Past Due [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans, net of unearned discounts 40,463    
Consumer Loan [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans, net of unearned discounts 425,179   444,474
Consumer Loan [Member] | Financing Receivables, 30 to 89 Days Past Due [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans, net of unearned discounts 6,867    
Bank Overdrafts | Financial Asset, Other than Financial Asset Acquired with Credit Deterioration | Nonperforming Financial Instruments [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Financing Receivable, Year One, Originated, Current Fiscal Year 1,800    
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year 3,800    
Commercial mortgage [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Loans, net of unearned discounts 7,171,220   7,165,220
Commitments to Extend Credit [Member] | Automobile Loan [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Unfunded balances 524,300    
Commitments to Extend Credit [Member] | Commercial and Industrial Total Energy Loans [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Unfunded balances 1,100,000    
Standby Letters of Credit [Member] | Automobile Loan [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Unfunded balances 20,200    
Standby Letters of Credit [Member] | Commercial and Industrial Total Energy Loans [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Unfunded balances 64,000    
Loans Receivable [Member]      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Interest Receivable $ 86,800   $ 86,800