Securities (Narrative) (Detail) (USD $) In Billions, unless otherwise specified	9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Schedule of Investments [Line Items]
Carrying value of pledged securities	$ 2.2	$ 2.4
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas	99.00%
Percentage of municipal bond portfolio issued by political subdivision or agencies within the state of Texas guaranteed by Texas permanent school fund	84.00%