Securities (Narrative) (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule Of Marketable Securities [Line Items]
Securities available for sale, at estimated fair value	$ 10,437,565	$ 11,269,591
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, pre tax	3,500	4,800
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax	2,800	3,800
Debt Securities, Held-to-maturity, Past Due	0
Debt Securities, Held-to-maturity, Allowance for Credit Loss	$ 160	0
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	77.80%
Debt Securities, Available-for-sale, Allowance for Credit Loss	$ 0	0
Debt Securities [Member]
Schedule Of Marketable Securities [Line Items]
Debt Securities, Held-to-maturity, Allowance for Credit Loss	160	0
Held-to-maturity Securities [Member]
Schedule Of Marketable Securities [Line Items]
Carry value of securities pledged	659,200	561,400
Accrued interest receivable	21,700	21,100
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
Securities Available For Sale [Member]
Schedule Of Marketable Securities [Line Items]
Carry value of securities pledged	4,400,000	3,400,000
Accrued interest receivable	111,000	$ 115,900
Guaranteed by the Texas PSF [Member] | Debt Securities [Member]
Schedule Of Marketable Securities [Line Items]
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0
Pre-Refunded [Member] | Debt Securities [Member]
Schedule Of Marketable Securities [Line Items]
Debt Securities, Held-to-maturity, Allowance for Credit Loss	$ 0