Unresolved Claims by Claimant (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended	3 Months Ended
	Dec. 31, 2009	Mar. 31, 2013 GSE number	Mar. 31, 2012 GSE number	Mar. 31, 2013 Private Label number	Mar. 31, 2012 Private Label number
Loss Contingencies
Balance, beginning of period		$ 48	$ 47	$ 19	$ 19
New demands		66	93	1	1
Loan paydowns/payoffs		(1)	(2)	(1)
Resolved claims		(71)	(78)	(14)	(2)
Balance, end of period		$ 42	$ 60	$ 5	$ 18
Balance, beginning of period		294,000,000	328,000,000	124,000,000	109,000,000
New demands	2	458,000,000	773,000,000	76,000,000	61,000,000
Loan paydowns/payoffs		(5,000,000)	(15,000,000)	(4,000,000)
Resolved claims		(484,000,000)	(672,000,000)	(163,000,000)	(58,000,000)
Balance, end of period		263,000,000	414,000,000	33,000,000	112,000,000