Securities - Additional Information (Detail) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Schedule Of Marketable Securities [Line Items]
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income Pre Tax	$ 2,800	$ 3,500
Net unamortized unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax	$ 2,200	2,800
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	76.90%
Debt Securities, Held-to-maturity, Allowance for Credit Loss	$ 0	0
Held-to-maturity Securities [Member]
Schedule Of Marketable Securities [Line Items]
Carry value of securities pledged	553,100	659,200
Accrued interest receivable	10,800	21,700
Available-for-sale Securities [Member]
Schedule Of Marketable Securities [Line Items]
Carry value of securities pledged	4,900,000	4,400,000
Accrued interest receivable	$ 69,000	$ 111,000