Consolidations and Transfers of Financial Assets Transfers of Financial Assets (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Transfers of Financial Assets [Line Items]
Unpaid principal balance portfolio securitizations	$ 225,100,000,000	$ 118,500,000,000	$ 120,000,000,000
Unconsolidated VIEs [Member]
Transfers of Financial Assets [Line Items]
Principal and interest received on retained interest	2,400,000,000	3,000,000,000	3,500,000,000
Unconsolidated VIEs [Member] | Fannie Mae Single-class MBS & Fannie Mae Megas [Member]
Transfers of Financial Assets [Line Items]
Unpaid principal balance	456,000,000	588,000,000
Fair value	504,000,000	654,000,000
Weighted-average coupon	6.20%	6.21%
Weighted-average loan age	6 years 5 months	5 years 5 months
Weighted-average maturity	22 years 6 months	23 years 6 months
Unconsolidated VIEs [Member] | REMICS & SMBS [Member]
Transfers of Financial Assets [Line Items]
Unpaid principal balance	8,667,000,000	12,697,000,000
Fair value	$ 9,818,000,000	$ 14,043,000,000
Weighted-average coupon	5.53%	5.86%
Weighted-average loan age	4 years 7 months	4 years 6 months
Weighted-average maturity	15 years 0 months	18 years 7 months