Affordable housing program assessments not remitted

No definition available.

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

- Definition

Affordable Housing Program Obligation, Portion

+ References

No definition available.

+ Details

Name:	fnm_AffordableHousingProgramObligationPortion
Namespace Prefix:	fnm_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the aggregated funding received to date from U.S. Treasury under the terms of the senior preferred stock purchase agreement.

+ References

No definition available.

+ Details

Name:	fnm_AggregateFundingReceivedFromUsTreasuryPursuantToSeniorPreferredStockPurchaseAgreement
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the aggregated liquidation preference of senior preferred stock issued and outstanding held by U.S. Treasury.

+ References

No definition available.

+ Details

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

- Definition

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.

+ References

No definition available.

+ Details

Name:	fnm_BasisPointsOfEachDollarOfUnpaidPrincipalBalance
Namespace Prefix:	fnm_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

Name:	fnm_IncreaseOfGuaranteeFeeRateResultingFromTemporaryPayrollTaxCutContinuationActOf2011
Namespace Prefix:	fnm_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Net Worth, Increase (Decrease)

+ References

No definition available.

+ Details

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

- Definition

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.

+ References

No definition available.

+ Details

Name:	fnm_PercentageOfCommonSharesAttributableToWarrantsIssuedToTreasuryAsPercentageToTotalDilutedCommonShares
Namespace Prefix:	fnm_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

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.

+ References

No definition available.

+ Details

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

- Definition

Represents total available funding from U.S. Treasury pursuant to the senior preferred stock agreement after 2012 if we do not have a positive net worth as of December 31, 2012. In the event we have a positive net worth as of December 31, 2012, then the amount of funding available after 2012 under the amended senior preferred stock purchase agreement will depend on the size of that positive net worth relative to the cumulative draws for net worth deficiencies attributable to periods during 2010, 2011, and 2012, as follows: 1) If our positive net worth as of December 31, 2012 is less than the cumulative draws for net worth deficiencies attributable to periods during 2010, 2011, and 2012, then the amount of available funding will be $124.8 billion less our positive net worth as of December 31, 2012. 2) If our positive net worth as of December 31, 2012 is greater than the cumulative draws for net worth deficiencies attributable to periods during 2010, 2011, and 2012, then the amount of available funding will be $124.8 billion less the cumulative draws attributable to periods during 2010, 2011, and 2012.

+ References

No definition available.

+ Details

Name:	fnm_TotalAvailableFundingFromTreasuryPursuantToTheSeniorPreferredStockAgreement
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Amount of net worth for mortgage banking as defined by regulatory framework.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)(1)
-URI https://asc.fasb.org/extlink&oid=6490092&loc=d3e47080-110998

+ Details

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

- Definition

Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of convertible preferred stock using the if-converted method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=124432515&loc=d3e3550-109257

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 42
-URI https://asc.fasb.org/extlink&oid=126958026&loc=d3e2029-109256

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 40
-Subparagraph (c)
-URI https://asc.fasb.org/extlink&oid=126958026&loc=d3e1930-109256

+ Details

Name:	us-gaap_IncrementalCommonSharesAttributableToConversionOfPreferredStock
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

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.

+ References

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 https://asc.fasb.org/extlink&oid=126954810&loc=d3e3213-108585

+ Details

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

- Definition

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.

+ References

No definition available.

+ Details

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

- Definition

Shares issuable for little or no cash consideration upon the satisfaction of certain conditions (contingently issuable shares) are considered outstanding common shares and included in the computation of basic Earnings Per Share as of the date that all necessary conditions have been satisfied (in essence, when issuance of the shares is no longer contingent). Outstanding common shares that are contingently returnable (that is, subject to recall) are treated in the same manner as contingently issuable shares. Contingently issuable shares include shares that (a) will be issued in the future upon the satisfaction of specified conditions, (b) have been placed in escrow and all or part must be returned if specified conditions are not met, or (c) have been issued but the holder must return all or part if specified conditions are not met. The number of contingently issuable shares is determined by relating the portion of time within a reporting period that these shares have been outstanding to the total time in that period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 45
-Paragraph 13
-URI https://asc.fasb.org/extlink&oid=126958026&loc=d3e2646-109256

+ Details

Name:	us-gaap_WeightedAverageNumberOfSharesContingentlyIssuable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Details

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

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=fnm_Fnm_SingleFamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

Summary of Significant Accounting Policies (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2022	Jun. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2022
Related Parties [Line Items]
Net worth	$ 58,800			$ 58,800
Increase in net worth	2,400	$ 4,600
TCCA fees	$ 850		$ 781	$ 2,515	$ 2,270
Basis points of each dollar of unpaid principal balance	0.042%			0.042%
Affordable Housing Program obligation	35.00%			35.00%
Weighted average number of contingently issuable shares (shares)	4,700		4,700	4,700	4,700
Incremental common shares attributable to dilutive effect of conversion of preferred stock (in shares)	26		26	26	26
US Treasury
Related Parties [Line Items]
Aggregate funding received from US Treasury pursuant to the senior preferred stock purchase agreement	$ 119,800			$ 119,800
Total available funding from US Treasury pursuant to the senior preferred stock agreement	113,900			113,900
Aggregate liquidation preference of senior preferred stock	$ 177,900			$ 177,900
Percentage of common shares attributable to warrants	79.90%			79.90%
Home Affordable Modification Program administrative expense reimbursements	$ 4		$ 4	$ 11	$ 13
TCCA, increase of guarantee fee rate	0.10%			0.10%
TCCA fees	$ 850		781	$ 2,515	2,270
US Treasury \| Other Expense
Related Parties [Line Items]
Affordable Housing Program assessments recognized	19		46	85	165
Affordable housing program assessments not remitted				85
US Treasury \| Single Family
Related Parties [Line Items]
Recognized TCCA fees that had not been remitted to Treasury as of period end	850			850
US Treasury \| Forecast
Related Parties [Line Items]
Aggregate liquidation preference of senior preferred stock						$ 180,300
Federal Housing Finance Agency
Related Parties [Line Items]
FHFA assessment fees	30		35	93	107
Common Securitization Solutions
Related Parties [Line Items]
Capital contributed	$ 13		$ 15	$ 51	$ 60