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, 2014	Mar. 31, 2013	Mar. 31, 2014 Unconsolidated VIEs [Member]	Mar. 31, 2013 Unconsolidated VIEs [Member]	Mar. 31, 2014 Unconsolidated VIEs [Member] Fannie Mae Single-class MBS & Fannie Megas [Member]	Dec. 31, 2013 Unconsolidated VIEs [Member] Fannie Mae Single-class MBS & Fannie Megas [Member]	Mar. 31, 2014 Unconsolidated VIEs [Member] REMICS & SMBS [Member]		Dec. 31, 2013 Unconsolidated VIEs [Member] REMICS & SMBS [Member]
Transfers of Financial Assets [Line Items]
Unpaid principal balance portfolio securitizations	$ 32,500,000,000	$ 69,400,000,000
Unpaid principal balance					331,000,000	349,000,000	6,879,000,000	[1]	6,899,000,000	[1]
Fair value					366,000,000	383,000,000	8,012,000,000	[1]	7,959,000,000	[1]
Weighted-average coupon					6.20%	6.21%	5.35%	[1]	5.36%	[1]
Weighted-average loan age					7 years 8 months	7 years 5 months	5 years 6 months	[1]	5 years 5 months	[1]
Weighted-average maturity					21 years 2 months	21 years 6 months	11 years 11 months	[1]	12 years 7 months	[1]
Principal and interest received on retained interests			$ 340,000,000	$ 459,000,000

[1] Consists of real estate mortgage investment conduits (“REMICs”) and stripped mortgage-backed securities (“SMBS”).