Name:	fnm_AggregateLiquidationPreferenceOfSeniorPreferredStock
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Represents the number of basis points for each dollar of the unpaid principal balance of its total new business purchases that Fannie Mae must set aside to fund the Housing Trust Fund and Capital Magnet Fund as required by the Federal Housing Finance Regulatory Reform Act of 2008, which was enacted on July 30, 2008.

No definition available.

Name:	fnm_BasisPointsofEachDollarofUnpaidPrincipalBalance
Namespace Prefix:	fnm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Fees paid to FHFA semi-annually as required by HERA Act for operating expenses and maintaining working capital.

No definition available.

Name:	fnm_FHFAassessmentfeesexpense
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Represents administrative expense reimbursement received from U.S. Treasury and Freddie Mac for expenses relating to U.S. Treasury's Home Affordable Modification Program.

No definition available.

Name:	fnm_HomeAffordableModificationProgramAdministrativeExpenseReimbursement
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Increase in guaranty fee on single-family residential mortgages required by the Temporary Payroll Tax Cut Continuation Act of 2011.

No definition available.

Name:	fnm_IncreaseOfGuaranteeFeeRateResultingFromTemporaryPayrollTaxCutContinuationActOf2011
Namespace Prefix:	fnm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

A liability due to the Treasury related to the increased guaranty fee result from the Temporary Payroll Tax Cut Continuation Act of 2011.

No definition available.

Name:	fnm_LiabilityToTreasuryRelatedToIncreaseInSingleFamilyGuarantyFeeResultingFromTemporaryPayrollTaxCutContinuationActOf2011
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Represents a percentage of common shares warrants issued to U.S. Treasury as a percentage of the total number of the company's diluted common shares.

No definition available.

Name:	fnm_PercentageOfCommonSharesAttributableToWarrantsIssuedToTreasuryAsPercentageToTotalDilutedCommonShares
Namespace Prefix:	fnm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Represents single-family guaranty fees related to Temporary Payroll Tax Cut Continuation Act of 2011 (“TCCA”) recognized during the period which is payable to U.S. Treasury.

No definition available.

Name:	fnm_SingleFamilyGuarantyFeeTemporaryPayrollTaxCutContinuationActOf2011
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Amount of expense for Affordable Housing Program (AHP) assessments required by the Federal Home Loan Bank Act of 1932.

No definition available.

Name:	us-gaap_AffordableHousingProgramAssessments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Amount of obligation related to Affordable Housing Program (AHP) assessments required by the Federal Home Loan Bank Act of 1932.

No definition available.

Name:	us-gaap_AffordableHousingProgramObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The cash outflow associated with the purchase of or advances to an equity method investments, which are investments in joint ventures and entities in which the entity has an equity ownership interest normally of 20 to 50 percent and exercises significant influence.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 13
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3213-108585

Name:	us-gaap_PaymentsToAcquireEquityMethodInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_RelatedPartyTransactionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=fnm_UsTreasuryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_OtherExpenseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_StatementScenarioAxis=srt_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=fnm_FederalHousingFinanceAgencyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=fnm_CommonSecuritizationSolutionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: