Securities - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Schedule Of Marketable Securities [Line Items]
AOCI, Debt Securities, Available-for-sale, Adjustment, Pre Tax	$ 1,300	$ 1,800
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax	$ 1,000	1,400
Percent Municipal Bond Portfolio Issued By Political Subdivision Or Agencies Within State Of Texas Guaranteed By Texas Permanent School Fund Or Have Been Pre-Refunded	73.30%
Debt Securities, Available-for-sale, Allowance for Credit Loss	$ 0	0
Held-to-maturity Securities [Member]
Schedule Of Marketable Securities [Line Items]
Carry Value of Securities Pledged	811,400	256,300
Accrued interest receivable	22,900	30,200
Available-for-sale Securities [Member]
Schedule Of Marketable Securities [Line Items]
Carry Value of Securities Pledged	4,800,000	8,000,000
Accrued interest receivable	$ 82,400	$ 140,600