Guarantees (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012
Guarantor Obligations [Line Items]
Liability Recorded	$ 153
Maximum Potential Undiscounted Future Payments	13,429
Standby Letters of Credit [Member]
Guarantor Obligations [Line Items]
Liability Recorded	52
Maximum Potential Undiscounted Future Payments	10,377
Recourse Agreement with FNMA [Member]
Guarantor Obligations [Line Items]
Liability Recorded	7
Maximum Potential Undiscounted Future Payments	1,003
Return Guarantee Agreement with LIHTC Investors [Member]
Guarantor Obligations [Line Items]
Liability Recorded	43
Maximum Potential Undiscounted Future Payments	43
Written Put Options [Member]
Guarantor Obligations [Line Items]
Liability Recorded	50
Maximum Potential Undiscounted Future Payments	2,003	[1]
Default Guarantees [Member]
Guarantor Obligations [Line Items]
Liability Recorded	1
Maximum Potential Undiscounted Future Payments	$ 3

[1] The maximum potential undiscounted future payments represent notional amounts of derivatives qualifying as guarantees.