Consolidations and Transfers of Financial Assets Transfers of Financial Assets (Details) (USD $)	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended			12 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Unconsolidated VIEs [Member]	Sep. 30, 2011 Unconsolidated VIEs [Member]	Sep. 30, 2012 Unconsolidated VIEs [Member]	Sep. 30, 2011 Unconsolidated VIEs [Member]	Sep. 30, 2012 Unconsolidated VIEs [Member] Fannie Mae Single-class MBS & Fannie Mae Megas [Member]	Dec. 31, 2011 Unconsolidated VIEs [Member] Fannie Mae Single-class MBS & Fannie Mae Megas [Member]	Sep. 30, 2012 Unconsolidated VIEs [Member] REMICS & SMBS [Member]		Dec. 31, 2011 Unconsolidated VIEs [Member] REMICS & SMBS [Member]
Transfers of Financial Assets [Line Items]
Unpaid principal balance portfolio securitizations	$ 65,500,000,000	$ 22,000,000,000	$ 153,600,000,000	$ 78,700,000,000
Unpaid principal balance									494,000,000	588,000,000	9,106,000,000	[1]	12,697,000,000	[1]
Fair value									551,000,000	654,000,000	10,277,000,000	[1]	14,043,000,000	[1]
Weighted-average coupon									6.21%	6.21%	5.60%	[1]	5.86%	[1]
Weighted-average loan age									6 years 1 month	5 years 5 months	4 years 6 months	[1]	4 years 6 months	[1]
Weighted-average maturity									22 years 9 months	23 years 6 months	15 years 6 months	[1]	18 years 7 months	[1]
Principal and interest received on retained interest					$ 563,000,000	$ 777,000,000	$ 1,900,000,000	$ 2,200,000,000

[1] Consists of Real Estate Mortgage Investment Conduits (“REMICs”) and stripped mortgage-backed securities (“SMBS”).