Loans (Narrative) (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2025 USD ($) Grade	Dec. 31, 2024 USD ($) Grade Rate	Dec. 31, 2023 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
U.S. dollar denominated loans and commitments to borrowers in Mexico	$ 0	$ 0
Commercial and industrial loans related to overdraft for correspondent bank customer	$ 13,800	14,600
Financing Receivable, Threshold Period Past Due	90 days
Loans, net of unearned discounts	$ 21,891,812	$ 20,754,813
Loan Receivable Variable Rate | Rate		8.50%
Number of days past due for payment of interest on loans, minimum	120 days
Texas Leading Index economic condition indicator	125.7	125.9
Financing Receivable, Change in Method, Credit Loss Expense (Reversal)		$ 7,200
Additional Funding Agreement Terms
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 0
Contractual Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	0	$ 31,302
Financing Receivable Modified Interest Rate After Modification | Rate		6.50%
Contractual Interest Rate Reduction | Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Modified Interest Rate After Modification | Rate		6.00%
variable rate of prime minus a spread
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Basis Spread on Variable Rate | Rate		(1.50%)
variable rate of prime plus a spread
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Basis Spread on Variable Rate | Rate		(0.50%)
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrued interest receivable	$ 90,600	$ 86,800
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total loans	5.00%
Loans, net of unearned discounts	$ 1,094,669	$ 1,128,895
Weighted Average Risk Grade | Grade	6.16	5.58
Commercial and Industrial Total Energy Loans [Member] | Risk Grade One To Eight [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,010,179	$ 1,111,319
Weighted Average Risk Grade | Grade	5.86	5.51
Automobile Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total loans	4.80%
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Excluding Accrued Interest, Impaired Interest Lost On Nonaccrual Loans	$ 4,400	$ 5,700	$ 4,000
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 Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 6,306,980	$ 6,109,532
Weighted Average Risk Grade | Grade	6.44	6.64
Commercial Portfolio Segment [Member] | Contractual Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 0	$ 0
Commercial Portfolio Segment [Member] | Risk Grade One To Eight [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 5,663,146	$ 5,553,757
Weighted Average Risk Grade | Grade	6.06	6.30
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 Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 10,310,810	$ 9,968,523
Weighted Average Risk Grade | Grade	7.29	7.35
Commercial Real Estate Owner Occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 3,987,913	$ 3,622,201
Weighted Average Risk Grade | Grade	7.08	7.22
Commercial Real Estate Owner Occupied | Contractual Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts		$ 31,302
Commercial Real Estate Owner Occupied | Risk Grade One To Eight [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 3,608,005	$ 3,238,091
Weighted Average Risk Grade | Grade	6.77	6.87
Commercial Real Estate Non-Owner Occupied
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 3,773,028	$ 3,543,019
Weighted Average Risk Grade | Grade	7.18	7.21
Financing Receivable, Change in Method, Credit Loss Expense (Reversal)		$ 6,200
Commercial Real Estate Non-Owner Occupied | Risk Grade One To Eight [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 3,494,323	$ 3,337,485
Weighted Average Risk Grade | Grade	6.96	7.05
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 2,549,869	$ 2,803,303
Weighted Average Risk Grade | Grade	7.78	7.70
Construction Loans [Member] | Risk Grade One To Eight [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 2,319,413	$ 2,387,930
Weighted Average Risk Grade | Grade	7.61	7.38
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 460,685	$ 444,474
Financing Receivable, Change in Method, Credit Loss Expense (Reversal)		$ 923
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	19,700
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	3,800
Commitments to Extend Credit [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	1,300,000
Commitments to Extend Credit [Member] | Automobile Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	570,500
Standby Letters of Credit [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	66,400
Standby Letters of Credit [Member] | Automobile Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	$ 20,000