Loans - Additional Information (Detail) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2026 USD ($)	Jun. 30, 2025 USD ($)	Jun. 30, 2026 USD ($)	Jun. 30, 2025 USD ($)	Dec. 31, 2025 USD ($)
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%		10.00%
U.S. dollar denominated loans and commitments to borrowers in Mexico	$ 0		$ 0		$ 0
Bank Overdrafts	17,400		17,400		13,800
Loans and Leases Receivable, Related Parties	315,400		315,400		316,100
Federal Home Loan Bank, Advances, General Debt Obligations, Disclosures, Collateral Pledged	20,800,000		20,800,000		19,600,000
Federal Home Loan Bank Borrowings, Fair Value Disclosure	0		0		0
Amorized Cost Of Loans Transfered to Loans Held For Sale	11,500		11,500
Financing Receivable Held For Sale Charge-Off	(2,100)		(2,100)
Loan, Held-for-Sale, Fair Value Disclosure	9,400		9,400
Loans, net of unearned discounts	$ 22,975,658		$ 22,975,658		$ 21,891,812
Texas Leading Index economic condition indicator	127.9		127.9		125.8
Payment Deferral
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 54,939	$ 4,977	$ 54,939	$ 4,977
Financing Receivables, 30 to 89 Days Past Due [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	$ 155,286		$ 155,286
Automobile Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total loans	4.70%		4.70%
Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Percentage of total loans	5.00%		5.00%
Loans, net of unearned discounts	$ 1,133,640		$ 1,133,640		$ 1,094,669
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	20,586		20,586
Non Accrual Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing Receivable, Excluding Accrued Interest, Impaired Interest Lost On Nonaccrual Loans	1,200	1,300	2,300	2,700
Total consumer real estate loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	4,069,982		4,069,982		3,718,668
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	25,205		25,205
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	6,325,658		6,325,658		6,306,980
Commercial Portfolio Segment [Member] | Additional Funding Agreement Terms
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	0		0
Commercial Portfolio Segment [Member] | Payment Deferral
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts		$ 3,101		$ 3,101
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	20,844		20,844
Consumer Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans, net of unearned discounts	457,802		457,802		460,685
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	5,572		5,572
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	7,700		7,700
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	3,700		3,700
Commitments to Extend Credit [Member] | Automobile Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	560,800		560,800
Commitments to Extend Credit [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	1,200,000		1,200,000
Standby Letters of Credit [Member] | Automobile Loan [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	19,800		19,800
Standby Letters of Credit [Member] | Commercial and Industrial Total Energy Loans [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Unfunded balances	93,700		93,700
Loans Receivable [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest Receivable	$ 89,300		$ 89,300		$ 90,600