Unresolved Claims by Claimant (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended	9 Months Ended
	Dec. 31, 2009	Sep. 30, 2012 Gse [Member] number	Sep. 30, 2012 Private Label [Member] number
Loss Contingencies [Line Items]
Balance, beginning of period		$ 47	$ 19
New demands		274	6
Loan paydowns/payoffs		(5)
Resolved claims		(261)	(6)
Balance, end of period		$ 55	$ 19
Balance, beginning of period		328,000,000	109,000,000
New demands	2	2,116,000,000	173,000,000
Loan paydowns/payoffs		(34,000,000)	(1,000,000)
Resolved claims		(2,092,000,000)	(157,000,000)
Balance, end of period		318,000,000	124,000,000