Allowance for Loan Losses Allowance Rollforward (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011		Dec. 31, 2011
Allowance for Loan Losses [Roll Forward]
Beginning balance	$ 63,375		$ 69,506		$ 72,156		$ 61,556
(Benefit) provision for loan losses	2,083	[1]	4,159	[1]	676	[1]	20,548	[1]
Charge-offs	(3,541)	[2],[3]	(4,113)	[2],[3]	(12,328)	[2],[3]	(16,484)	[2],[3]
Recoveries	350		883		1,321		4,734
Transfers	0	[4]	0	[4]	0	[4]	0	[4]
Other	745	[5]	1,000	[5]	1,187	[5]	1,081	[5]
Ending balance	63,012	[6],[7]	71,435	[6]	63,012	[6],[7]	71,435	[6]
Allowance for loan losses for acquired credit-impaired loans	376	[7]	334		376	[7]	334		375
Interest Receivable [Member]
Allowance for Loan Losses [Roll Forward]
Charge-offs	(198)		(289)		(709)		(1,100)
Fannie Mae [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	52,082		55,966		57,309		48,530
(Benefit) provision for loan losses	3,029	[1]	(196)	[1]	1,083	[1]	10,003	[1]
Charge-offs	(3,309)	[2],[3]	(3,853)	[2],[3]	(11,625)	[2],[3]	(15,018)	[2],[3]
Recoveries	323		848		1,185		3,197
Transfers	1,378	[4]	1,770	[4]	5,195	[4]	7,739	[4]
Other	711	[5]	863	[5]	1,067	[5]	947	[5]
Ending balance	54,214		55,398		54,214		55,398
Fannie Mae [Member] | Interest Receivable [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance									2,200
Ending balance	1,600				1,600				2,200
Consolidated Trusts [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	11,293		13,540		14,847		13,026
(Benefit) provision for loan losses	(946)	[1]	4,355	[1]	(407)	[1]	10,545	[1]
Charge-offs	(232)	[2],[3]	(260)	[2],[3]	(703)	[2],[3]	(1,466)	[2],[3]
Recoveries	27		35		136		1,537
Transfers	(1,378)	[4]	(1,770)	[4]	(5,195)	[4]	(7,739)	[4]
Other	34	[5]	137	[5]	120	[5]	134	[5]
Ending balance	8,798		16,037		8,798		16,037
Consolidated Trusts [Member] | Interest Receivable [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance									336
Ending balance	218				218				336
Single-Family [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	62,134		68,027		70,633		59,980
(Benefit) provision for loan losses	2,182	[1]	4,083	[1]	888	[1]	20,372	[1]
Charge-offs	(3,513)	[2],[3]	(4,062)	[2],[3]	(12,112)	[2],[3]	(16,232)	[2],[3]
Recoveries	350		883		1,321		4,734
Transfers	0	[4]	0	[4]	0	[4]	0	[4]
Other	744	[5]	997	[5]	1,167	[5]	1,074	[5]
Ending balance	61,897	[7]	69,928		61,897	[7]	69,928
Allowance for loan losses for acquired credit-impaired loans	375	[7]			375	[7]			374
Single-Family [Member] | Interest Receivable [Member]
Allowance for Loan Losses [Roll Forward]
Charge-offs	(196)		(279)		(682)		(1,100)
Single-Family [Member] | Fannie Mae [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	51,322		54,949		56,294		47,377
(Benefit) provision for loan losses	3,104	[1]	(235)	[1]	1,260	[1]	9,962	[1]
Charge-offs	(3,281)	[2],[3]	(3,802)	[2],[3]	(11,409)	[2],[3]	(14,766)	[2],[3]
Recoveries	323		848		1,185		3,197
Transfers	1,372	[4]	1,764	[4]	5,172	[4]	7,676	[4]
Other	710	[5]	860	[5]	1,048	[5]	938	[5]
Ending balance	53,550		54,384		53,550		54,384
Single-Family [Member] | Consolidated Trusts [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	10,812		13,078		14,339		12,603
(Benefit) provision for loan losses	(922)	[1]	4,318	[1]	(372)	[1]	10,410	[1]
Charge-offs	(232)	[2],[3]	(260)	[2],[3]	(703)	[2],[3]	(1,466)	[2],[3]
Recoveries	27		35		136		1,537
Transfers	(1,372)	[4]	(1,764)	[4]	(5,172)	[4]	(7,676)	[4]
Other	34	[5]	137	[5]	119	[5]	136	[5]
Ending balance	8,347		15,544		8,347		15,544
Multifamily [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	1,241		1,479		1,523		1,576
(Benefit) provision for loan losses	(99)	[1]	76	[1]	(212)	[1]	176	[1]
Charge-offs	(28)	[2],[3]	(51)	[2],[3]	(216)	[2],[3]	(252)	[2],[3]
Transfers	0	[4]	0	[4]	0	[4]	0	[4]
Other	1	[5]	3	[5]	20	[5]	7	[5]
Ending balance	1,115	[7]	1,507		1,115	[7]	1,507
Allowance for loan losses for acquired credit-impaired loans	1	[7]			1	[7]			1
Multifamily [Member] | Interest Receivable [Member]
Allowance for Loan Losses [Roll Forward]
Charge-offs	(2)		(10)		(27)		(34)
Multifamily [Member] | Fannie Mae [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	760		1,017		1,015		1,153
(Benefit) provision for loan losses	(75)	[1]	39	[1]	(177)	[1]	41	[1]
Charge-offs	(28)	[2],[3]	(51)	[2],[3]	(216)	[2],[3]	(252)	[2],[3]
Transfers	6	[4]	6	[4]	23	[4]	63	[4]
Other	1	[5]	3	[5]	19	[5]	9	[5]
Ending balance	664		1,014		664		1,014
Multifamily [Member] | Consolidated Trusts [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	481		462		508		423
(Benefit) provision for loan losses	(24)	[1]	37	[1]	(35)	[1]	135	[1]
Charge-offs	0	[2],[3]	0	[2],[3]	0	[2],[3]	0	[2],[3]
Transfers	(6)	[4]	(6)	[4]	(23)	[4]	(63)	[4]
Other	0	[5]	0	[5]	1	[5]	(2)	[5]
Ending balance	$ 451		$ 493		$ 451		$ 493

[1] Provision (benefit) for loan losses is included in (provision) benefit for credit losses in our condensed consolidated statements of operations and comprehensive income (loss).

[2] Total charge-offs include accrued interest of $198 million and $289 million for the three months ended September 30, 2012 and 2011, respectively, and $709 million and $1.1 billion for the nine months ended September 30, 2012 and 2011, respectively. Single-family charge-offs include accrued interest of $196 million and $279 million for the three months ended September 30, 2012 and 2011, respectively, and $682 million and $1.1 billion for the nine months ended September 30, 2012 and 2011, respectively. Multifamily charge-offs include accrued interest of $2 million and $10 million for the three months ended September 30, 2012 and 2011, respectively, and $27 million and $34 million for the nine months ended September 30, 2012 and 2011, respectively.

[3] While we purchase the substantial majority of loans that are four or more months delinquent from our MBS trusts, we do not exercise this option to purchase loans during a forbearance period. Accordingly, charge-offs of consolidated trusts generally represent loans that remained in our consolidated trusts at the time of default.

[4] Includes transfers from trusts for delinquent loan purchases.

[5] Amounts represent the net activity recorded in our allowances for accrued interest receivable and preforeclosure property taxes and insurance receivable from borrowers. The provision for credit losses, charge-offs, recoveries and transfer activity included in this table reflects all changes for both the allowance for loan losses and the valuation allowances for accrued interest and preforeclosure property taxes and insurance receivable that relate to the mortgage loans.

[6] Total allowance for loan losses includes $376 million and $334 million as of September 30, 2012 and 2011, respectively, for acquired credit-impaired loans.

[7] As of September 30, 2012, we began classifying as TDRs loans to certain borrowers who have received bankruptcy relief. These loans had a recorded investment of $22.9 billion and a related allowance of $1.1 billion as of September 30, 2012.