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, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Unconsolidated VIEs [Member]	Sep. 30, 2012 Unconsolidated VIEs [Member]	Sep. 30, 2013 Unconsolidated VIEs [Member]	Sep. 30, 2012 Unconsolidated VIEs [Member]	Sep. 30, 2013 Unconsolidated VIEs [Member] Fannie Mae Single-class MBS & Fannie Mae Megas [Member]	Dec. 31, 2012 Unconsolidated VIEs [Member] Fannie Mae Single-class MBS & Fannie Mae Megas [Member]	Sep. 30, 2013 Unconsolidated VIEs [Member] REMICS & SMBS [Member]		Dec. 31, 2012 Unconsolidated VIEs [Member] REMICS & SMBS [Member]
Transfers of Financial Assets [Line Items]
Unpaid principal balance portfolio securitizations	$ 52,200,000,000	$ 65,500,000,000	$ 184,000,000,000	$ 153,600,000,000
Unpaid principal balance									366,000,000	456,000,000	7,388,000,000	[1]	8,667,000,000	[1]
Fair value									403,000,000	504,000,000	8,372,000,000	[1]	9,818,000,000	[1]
Weighted-average coupon									6.21%	6.20%	5.20%	[1]	5.53%	[1]
Weighted-average loan age									7 years 2 months	6 years 5 months	5 years 0 months	[1]	4 years 7 months	[1]
Weighted-average maturity									21 years 8 months	22 years 6 months	12 years 11 months	[1]	15 years 0 months	[1]
Principal and interest received on retained interest					$ 434,000,000	$ 563,000,000	$ 1,400,000,000	$ 1,900,000,000

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