Guarantee Arrangements and Pledged Assets - Trend in Repurchase Demands Received on Loans Sold to GSEs and Other Third Parties by Loan Origination Vintage (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Commitments And Guarantee Obligations [Line Items]
Pre- 2004	$ 2	$ 1	$ 5	$ 4
2004	5	1	16	10
2005	6	6	19	20
2006	20	11	65	35
2007	51	34	170	104
2008	28	15	106	73
Post 2008	4	14	15	59
Total repurchase demands received	$ 116	$ 82	$ 396	$ 305