Unresolved Claims by Claimant (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2009	Dec. 31, 2012 Gse [Member] number	Dec. 31, 2011 Gse [Member] number	Dec. 31, 2012 Private Label [Member] number	Dec. 31, 2011 Private Label [Member] number
Loss Contingencies [Line Items]
Balance, beginning of period		$ 47	$ 150	$ 19	$ 11
New demands		333	328	7	22
Loan paydowns/payoffs		(7)	(3)
Resolved claims		(325)	(428)	(7)	(14)
Balance, end of period		$ 48	$ 47	$ 19	$ 19
Balance, beginning of period		328,000,000	845,000,000	109,000,000	71,000,000
New demands	2	2,519,000,000	2,050,000,000	230,000,000	107,000,000
Loan paydowns/payoffs		(42,000,000)	(21,000,000)	(2,000,000)	(2,000,000)
Resolved claims		(2,511,000,000)	(2,546,000,000)	(213,000,000)	(67,000,000)
Balance, end of period		294,000,000	328,000,000	124,000,000	109,000,000