Securities (Narrative) (Detail) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 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	$ 1,200	$ 1,800
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax	909	1,400
Debt Securities, Held-to-maturity, 90 Days or More Past Due, Still Accruing	0
Securities held to maturity, net of allowance for credit losses of $310 in 2023 and $158 in 2022	$ 310	158
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	73.50%
Debt Securities, Available-for-sale, Allowance for Credit Loss	$ 0	0
Debt Securities [Member]
Schedule Of Marketable Securities [Line Items]
Securities held to maturity, net of allowance for credit losses of $310 in 2023 and $158 in 2022	310	158	$ 158	$ 160
Held-to-maturity Securities [Member]
Schedule Of Marketable Securities [Line Items]
Carry value of securities pledged	1,000,000	256,300
Accrued interest receivable	40,900	30,200
Guaranteed by the Texas PSF [Member] | Debt Securities [Member]
Schedule Of Marketable Securities [Line Items]
Securities held to maturity, net of allowance for credit losses of $310 in 2023 and $158 in 2022	0
Pre-Refunded [Member] | Debt Securities [Member]
Schedule Of Marketable Securities [Line Items]
Securities held to maturity, net of allowance for credit losses of $310 in 2023 and $158 in 2022	0
Available-for-sale Securities [Member]
Schedule Of Marketable Securities [Line Items]
Carry value of securities pledged	6,100,000	8,000,000
Accrued interest receivable	$ 114,900	$ 140,600