Guarantee Arrangements and Pledged Assets - Summary of Change in Estimated Repurchase Liability for Loans Sold to GSEs and Other Third Parties (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
Resale Agreement Counterparty [Line Items]
Balance at beginning of period	$ 222	$ 237	$ 237	$ 262
Increase in liability recorded through earnings	28	1	81	41
Realized losses	(34)	(29)	(102)	(94)
Balance at end of period	$ 216	$ 209	$ 216	$ 209