Securities - Additional Information (Detail) (USD $)	9 Months Ended
	Sep. 30, 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	96.10%
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas guaranteed by Texas permanent school fund	76.40%
Carrying value of pledged securities	$ 2,500,000,000	$ 2,700,000,000
Available for Sale, Estimated Fair Value	5,569,791,000	6,203,299,000
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 unrealized gain on transferred securities included in accumulated other comprehensive income before Tax	138,800,000
Net unrealized gain on transferred securities included in accumulated other comprehensive income after Tax	90,200,000
Reclassified To Held To Maturity [Member]
Schedule Of Marketable Securities [Line Items]
Available for Sale, Estimated Fair Value		$ 2,300,000,000