Loans (Narrative) (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2024 USD ($) Grade Rate	Dec. 31, 2023 USD ($) Grade Rate	Dec. 31, 2022 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	$ 14,600	13,400
Financing Receivable, Threshold Period Past Due	90 days
Loans, net of unearned discounts	$ 20,754,813	$ 18,824,251
Loans Receivable, Basis Spread on Variable Rate | Rate	(0.50%)	(1.75%)
Loan Receivable Variable Rate | Rate	8.50%	10.25%
Number of days past due for payment of interest on loans, minimum	120 days
Texas Leading Index economic condition indicator	125.9	127.2
Financing Receivable, Change in Method, Credit Loss Expense (Reversal)	$ 7,200	$ (45,000)
Prime Rate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans Receivable, Basis Spread on Variable Rate | Rate	(1.50%)
Additional Funding Agreement Terms
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 44,100
Contractual Interest Rate Reduction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Modified Interest Rate After Modification | Rate	6.50%	6.74%
Contractual Interest Rate Reduction | Minimum [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable Modified Interest Rate After Modification | Rate	6.00%
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accrued interest receivable	$ 86,800	$ 90,800
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total loans	5.40%
Loans, net of unearned discounts	$ 1,128,895	$ 936,737
Weighted Average Risk Grade | Grade	5.58	6.05
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	$ 1,111,319	$ 871,221
Weighted Average Risk Grade | Grade	5.51	5.73
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Excluding Accrued Interest, Impaired Interest Lost On Nonaccrual Loans	$ 5,700	$ 4,000	$ 1,700
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	$ 6,109,532	$ 5,967,182
Weighted Average Risk Grade | Grade	6.64	6.60
Commercial Portfolio Segment [Member] | Risk Grade One To Eight [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 5,553,757	$ 5,507,878
Weighted Average Risk Grade | Grade	6.30	6.32
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	$ 9,968,523	$ 8,982,644
Weighted Average Risk Grade | Grade	7.35	7.24
Commercial Real Estate Buildings Land And Other Financing Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 7,704,447	$ 7,301,920
Weighted Average Risk Grade | Grade	7.25	7.20
Commercial Real Estate Buildings Land And Other Financing Receivable [Member] | Risk Grade One To Eight [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 7,103,502	$ 6,895,358
Weighted Average Risk Grade | Grade	7.01	7.03
Construction Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 2,264,076	$ 1,680,724
Weighted Average Risk Grade | Grade	7.71	7.45
Construction Loans [Member] | Risk Grade One To Eight [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 1,860,004	$ 1,526,086
Weighted Average Risk Grade | Grade	7.31	7.27
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 444,474	$ 476,962
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	24,200
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	$ 5,500
Automobile Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total loans	6.00%
Commercial mortgage [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 7,165,220	$ 6,746,709
Financing Receivable, Change in Method, Credit Loss Expense (Reversal)	6,200
Commitments to Extend Credit [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	1,000,000
Commitments to Extend Credit [Member] | Automobile Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	535,500
Standby Letters of Credit [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	62,800
Standby Letters of Credit [Member] | Automobile Loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	$ 20,200