Securities (Narrative) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Schedule Of Marketable Securities [Line Items]
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas	99.70%
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas guaranteed by Texas permanent school fund	69.10%
Carry value of securities pledged	$ 3,900,000	$ 3,800,000
Securities available for sale, at estimated fair value	11,269,591	11,387,321
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, pre tax	4,800	2,700
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax	3,800	$ 2,200
Other than temporary impairment, credit losses recognized in earnings, credit losses on debt securities held	0
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