Securities (Narrative) (Detail) - USD ($) $ in Thousands	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Schedule Of Marketable Securities [Line Items]
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, pre tax	$ 2,500	$ 3,500
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax	2,000	2,800
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 $158 in 2021 and $160 in 2020	$ 158	160
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	76.60%
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 $158 in 2021 and $160 in 2020	158	160	$ 0
Held-to-maturity Securities [Member]
Schedule Of Marketable Securities [Line Items]
Carry value of securities pledged	642,300	659,200
Accrued interest receivable	18,400	21,700
Reclassified To Held To Maturity [Member]
Schedule Of Marketable Securities [Line Items]
Securities available for sale, at estimated fair value			377,800
Available for sale securities reclassified to held to maturity, unrealized gain			3,300
Available for sale securities reclassified to held to maturity, unrealized gain net of tax			$ 2,600
Available-for-sale Securities [Member]
Schedule Of Marketable Securities [Line Items]
Carry value of securities pledged	5,800,000	4,400,000
Accrued interest receivable	120,500	$ 111,000
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 $158 in 2021 and $160 in 2020	0
Pre-Refunded [Member] | Debt Securities [Member]
Schedule Of Marketable Securities [Line Items]
Securities held to maturity, net of allowance for credit losses of $158 in 2021 and $160 in 2020	$ 0