Securities (Narrative) (Detail) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Schedule Of Marketable Securities [Line Items]
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 $500 at 2025 and $310 at 2024	$ 500	$ 310
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	71.50%
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 $500 at 2025 and $310 at 2024	500	310	$ 310	$ 158
Held-to-maturity Securities [Member]
Schedule Of Marketable Securities [Line Items]
Carry value of securities pledged	1,400,000	1,400,000
Accrued interest receivable	37,200	37,800
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 $500 at 2025 and $310 at 2024	0
Pre-Refunded [Member] | Debt Securities [Member]
Schedule Of Marketable Securities [Line Items]
Securities held to maturity, net of allowance for credit losses of $500 at 2025 and $310 at 2024	0
Available-for-sale Securities [Member]
Schedule Of Marketable Securities [Line Items]
Carry value of securities pledged	6,600,000	6,200,000
Accrued interest receivable	$ 122,500	$ 104,900