Fair Value Fair Value of Financial Instruments (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Carrying Value [Member]
Financial assets [Abstract]
Cash and cash equivalents and restricted cash	$ 62,309	$ 89,036
Federal funds sold and securities purchased under agreements to resell or similar arrangements	40,300	32,500
Trading securities	32,860	40,695
Available-for-sale securities	41,000	63,181
Mortgage loans held for sale	998	464
Mortgage loans held for investment, at fair value	3,012,182	2,948,942
Advances to lenders	3,633	7,592
Derivatives assets at fair value	2,019	435
Guaranty assets and buy-ups	271	327
Total assets	3,195,572	3,183,172
Financial liabilities [Abstract]
Derivative liabilities at fair value	2,361	705
Guaranty obligations	502	599
Total financial liabilities	3,242,984	3,190,821
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial assets [Abstract]
Cash and cash equivalents and restricted cash	44,809	75,786
Federal funds sold and securities purchased under agreements to resell or similar arrangements	0	0
Trading securities	16,396	17,950
Available-for-sale securities	0	0
Mortgage loans held for sale	0	0
Mortgage loans held for investment, at fair value	0	0
Advances to lenders	0	0
Derivatives assets at fair value	0	0
Guaranty assets and buy-ups	0	0
Total assets	61,205	93,736
Financial liabilities [Abstract]
Derivative liabilities at fair value	0	0
Guaranty obligations	0	0
Total financial liabilities	0	0
Significant Other Observable Inputs (Level 2) [Member]
Financial assets [Abstract]
Cash and cash equivalents and restricted cash	17,500	13,250
Federal funds sold and securities purchased under agreements to resell or similar arrangements	40,300	32,500
Trading securities	13,779	20,459
Available-for-sale securities	22,021	38,147
Mortgage loans held for sale	223	267
Mortgage loans held for investment, at fair value	2,617,991	2,567,022
Advances to lenders	3,050	6,936
Derivatives assets at fair value	11,202	16,051
Guaranty assets and buy-ups	0	0
Total assets	2,726,066	2,694,632
Financial liabilities [Abstract]
Derivative liabilities at fair value	13,152	22,590
Guaranty obligations	0	0
Total financial liabilities	3,280,184	3,347,367
Significant Unobservable Inputs (Level 3) [Member]
Financial assets [Abstract]
Cash and cash equivalents and restricted cash	0	0
Federal funds sold and securities purchased under agreements to resell or similar arrangements	0	0
Trading securities	2,685	2,286
Available-for-sale securities	18,979	25,034
Mortgage loans held for sale	802	208
Mortgage loans held for investment, at fair value	393,817	466,594
Advances to lenders	548	572
Derivatives assets at fair value	92	175
Guaranty assets and buy-ups	684	692
Total assets	417,607	495,561
Financial liabilities [Abstract]
Derivative liabilities at fair value	118	161
Guaranty obligations	2,472	3,113
Total financial liabilities	18,931	23,984
Netting Adjustment [Member]
Financial assets [Abstract]
Cash and cash equivalents and restricted cash	0	0
Federal funds sold and securities purchased under agreements to resell or similar arrangements	0	0
Trading securities	0	0
Available-for-sale securities	0	0
Mortgage loans held for sale	0	0
Mortgage loans held for investment, at fair value	0	0
Advances to lenders	0	0
Derivatives assets at fair value	(9,275)	(15,791)
Guaranty assets and buy-ups	0	0
Total assets	(9,275)	(15,791)
Financial liabilities [Abstract]
Derivative liabilities at fair value	(10,909)	(22,046)
Guaranty obligations	0	0
Total financial liabilities	(10,909)	(22,046)
Estimated Fair Value [Member]
Financial assets [Abstract]
Cash and cash equivalents and restricted cash	62,309	89,036
Federal funds sold and securities purchased under agreements to resell or similar arrangements	40,300	32,500
Trading securities	32,860	40,695
Available-for-sale securities	41,000	63,181
Mortgage loans held for sale	1,025	475
Mortgage loans held for investment, at fair value	3,011,808	3,033,616
Advances to lenders	3,598	7,508
Derivatives assets at fair value	2,019	435
Guaranty assets and buy-ups	684	692
Total assets	3,195,603	3,268,138
Financial liabilities [Abstract]
Derivative liabilities at fair value	2,361	705
Guaranty obligations	2,472	3,113
Total financial liabilities	3,288,206	3,349,305
Fannie Mae [Member]
Financial liabilities [Abstract]
Long-term debt	684	793
Fannie Mae [Member] | Carrying Value [Member]
Financial assets [Abstract]
Mortgage loans held for investment, at fair value	271,923	305,025
Financial liabilities [Abstract]
Short-term debt	77,813	105,233
Long-term debt	487,297	510,631
Fannie Mae [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial assets [Abstract]
Mortgage loans held for investment, at fair value	0	0
Financial liabilities [Abstract]
Short-term debt	0	0
Long-term debt	0	0
Fannie Mae [Member] | Significant Other Observable Inputs (Level 2) [Member]
Financial assets [Abstract]
Mortgage loans held for investment, at fair value	33,998	39,018
Financial liabilities [Abstract]
Short-term debt	77,830	105,253
Long-term debt	498,046	534,516
Fannie Mae [Member] | Significant Unobservable Inputs (Level 3) [Member]
Financial assets [Abstract]
Mortgage loans held for investment, at fair value	223,365	232,170
Financial liabilities [Abstract]
Short-term debt	0	0
Long-term debt	962	1,056
Fannie Mae [Member] | Netting Adjustment [Member]
Financial assets [Abstract]
Mortgage loans held for investment, at fair value	0	0
Financial liabilities [Abstract]
Short-term debt	0	0
Long-term debt	0	0
Fannie Mae [Member] | Estimated Fair Value [Member]
Financial assets [Abstract]
Mortgage loans held for investment, at fair value	257,363	271,188
Financial liabilities [Abstract]
Short-term debt	77,830	105,253
Long-term debt	499,008	535,572
Consolidated Trusts [Member]
Financial assets [Abstract]
Mortgage loans held for investment, at fair value	13,877	10,800
Financial liabilities [Abstract]
Long-term debt	14,414	11,647
Consolidated Trusts [Member] | Carrying Value [Member]
Financial assets [Abstract]
Mortgage loans held for investment, at fair value	2,740,259	2,643,917
Financial liabilities [Abstract]
Short-term debt	2,297	3,483
Long-term debt	2,672,714	2,570,170
Consolidated Trusts [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial assets [Abstract]
Mortgage loans held for investment, at fair value	0	0
Financial liabilities [Abstract]
Short-term debt	0	0
Long-term debt	0	0
Consolidated Trusts [Member] | Significant Other Observable Inputs (Level 2) [Member]
Financial assets [Abstract]
Mortgage loans held for investment, at fair value	2,583,993	2,528,004
Financial liabilities [Abstract]
Short-term debt	0	0
Long-term debt	2,691,156	2,685,008
Consolidated Trusts [Member] | Significant Unobservable Inputs (Level 3) [Member]
Financial assets [Abstract]
Mortgage loans held for investment, at fair value	170,452	234,424
Financial liabilities [Abstract]
Short-term debt	2,297	3,483
Long-term debt	13,082	16,171
Consolidated Trusts [Member] | Netting Adjustment [Member]
Financial assets [Abstract]
Mortgage loans held for investment, at fair value	0	0
Financial liabilities [Abstract]
Short-term debt	0	0
Long-term debt	0	0
Consolidated Trusts [Member] | Estimated Fair Value [Member]
Financial assets [Abstract]
Mortgage loans held for investment, at fair value	2,754,445	2,762,428
Financial liabilities [Abstract]
Short-term debt	2,297	3,483
Long-term debt	$ 2,704,238	$ 2,701,179