Securities - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Schedule Of Marketable Securities [Line Items]
AOCI, Debt Securities, Available-for-sale, Adjustment, Pre Tax	$ 2,300		$ 2,500
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax	$ 1,800		2,000
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	75.70%
Debt Securities, Available-for-sale, Allowance for Credit Loss	$ 0		0
Proceeds from Sale of Debt Securities, Available-for-sale	0	$ 0
Proceeds from Sale of Held-to-maturity Securities	0	$ 0
Held-to-maturity Securities [Member]
Schedule Of Marketable Securities [Line Items]
Carry value of securities pledged	252,000		642,300
Accrued interest receivable	10,000		18,400
Available-for-sale Securities [Member]
Schedule Of Marketable Securities [Line Items]
Carry value of securities pledged	4,800,000		5,800,000
Accrued interest receivable	$ 80,800		$ 120,500