Financial Guarantees and Maximum Recovery (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Financial Guarantees and Maximum Recovery [Line Items]
Maximum Exposure	34,957	$ 35,965
Guaranty Obligation	367	382
Maximum Recovery	14,007fnm_MaximumRecovery	14,222fnm_MaximumRecovery
Fair Value of Guaranty obligations in Investments in Securities	769	797
Unconsolidated VIEs [Member] \| Unconsolidated Fannie Mae MBS [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Maximum Exposure	16,703	17,184
Guaranty Obligation	209	214
Maximum Recovery	9,576	9,775
Unconsolidated VIEs [Member] \| Other guaranty arrangements [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Maximum Exposure	18,254	18,781
Guaranty Obligation	158	168
Maximum Recovery	4,431	$ 4,447
Minimum [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Remaining contractual terms of our guarantees	30 days
Maximum [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Remaining contractual terms of our guarantees	38 years

Represents the original contractual term of guarantees.

No definition available.

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- Details

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- Definition

The fair value of our guaranty obligations associated with the Fannie Mae MBS included in “Investments in securities”

+ References

No definition available.

+ Details

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- Definition

Represents the maximum amount we could recover through available credit enhancements and recourse with third parties on guarantees recognized in our on and off balance sheets. Recoverability of such credit enhancements and recourse is subject to, among other factors, our mortgage insurers' and financial guarantors' ability to meet their obligations to us.

+ References

No definition available.

+ Details

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- Definition

The current carrying amount of the liability for the freestanding or embedded guarantor's obligations under the guarantee or each group of similar guarantees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 460

-SubTopic 10

-Section 50

-Paragraph 4

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248

+ Details

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- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 460

-SubTopic 10

-Section 50

-Paragraph 4

-Subparagraph (b)(1)

-URI http://asc.fasb.org/extlink&oid=6851643&loc=d3e12069-110248

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