Securities (Narrative) (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule Of Marketable Securities [Line Items]
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas	97.40%
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas guaranteed by Texas permanent school fund	64.90%
Carrying value of pledged securities	$ 3,000,000,000	$ 3,000,000,000
Securities available for sale, at estimated fair value	8,461,254,000	5,895,436,000
Available for sale securities reclassified to held to maturity, unrealized gain			165,700,000
Available for sale securities reclassified to held to maturity, unrealized gain net of tax			107,700,000
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, pre tax	93,900,000	129,300,000
Net unamortized, unrealized gain on transferred securities included in accumulated other comprehensive income, net of tax	61,000,000	84,100,000
Reclassified To Held To Maturity [Member]
Schedule Of Marketable Securities [Line Items]
Securities available for sale, at estimated fair value			$ 2,300,000,000