Consolidations and Transfers of Financial Assets Transfers of Financial Assets (Details) (USD $)	3 Months Ended					12 Months Ended	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Mar. 31, 2013 Unconsolidated VIEs [Member]	Mar. 31, 2012 Unconsolidated VIEs [Member]	Mar. 31, 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]	Mar. 31, 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	$ 69,400,000,000	$ 41,700,000,000
Unpaid principal balance					428,000,000	456,000,000	8,030,000,000	[1]	8,667,000,000	[1]
Fair value					475,000,000	504,000,000	9,229,000,000	[1]	9,818,000,000	[1]
Weighted-average coupon					6.21%	6.20%	5.51%	[1]	5.53%	[1]
Weighted-average loan age					6 years 8 months	6 years 5 months	5 years 0 months	[1]	4 years 7 months	[1]
Weighted-average maturity					22 years 2 months	22 years 6 months	14 years 1 month	[1]	15 years 0 months	[1]
Principal and interest received on retained interest			$ 459,000,000	$ 694,000,000

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