Financial Guarantees (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2025	Dec. 31, 2024
Financial Guarantees and Maximum Recovery [Line Items]
Contractual terms of our guarantees	29 years
Maximum Exposure	$ 11,373	$ 11,398
Guaranty Obligation	64	69
Maximum recovery	4,365	4,386
Unconsolidated Fannie Mae MBS \| Unconsolidated VIEs
Financial Guarantees and Maximum Recovery [Line Items]
Maximum Exposure	2,365	2,484
Guaranty Obligation	13	13
Maximum recovery	2,319	2,432
Other guaranty arrangements \| Unconsolidated VIEs
Financial Guarantees and Maximum Recovery [Line Items]
Maximum Exposure	9,008	8,914
Guaranty Obligation	51	56
Maximum recovery	2,046	1,954
Mortgage-Backed Security, Issued by US Government-Sponsored Enterprise \| Unconsolidated VIEs
Financial Guarantees and Maximum Recovery [Line Items]
Single security, Freddie Mac issued securities backing unconsolidated Fannie Mae securities	$ 192,200	$ 200,100

[Line Items] for Financial Guarantees and Maximum Recovery [Table]

No definition available.

Name:	fnm_FinancialGuaranteesandMaximumRecoveryLineItems
Namespace Prefix:	fnm_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Portion of Freddie Mac securities backing commingled Fannie Mae-issued structured securities.

+ References

No definition available.

+ Details

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

- 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

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

- 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/disclosureRef
-Topic 460
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482425/460-10-50-4

+ Details

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

- 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/disclosureRef
-Topic 460
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482425/460-10-50-4

+ Details

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

- Definition

Term of guarantee or group of similar guarantees, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 460
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482425/460-10-50-4

+ Details

Name:	us-gaap_GuarantorObligationTerm1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_GuaranteeObligationsByNatureAxis=fnm_NonconsolidatedFannieMaesecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidatedEntitiesAxis=us-gaap_VariableInterestEntityNotPrimaryBeneficiaryMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_GuaranteeObligationsByNatureAxis=us-gaap_GuaranteeTypeOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: