Name:	fnm_CurrentDebtServiceCoverageRatioHigherRiskLoans1
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Represents the percentage of unpaid principal balance comprised of loans 30 to 59 days past due. Calculated based on the aggregate unpaid principal balance for the category indicated, divided by the respective book of business.

No definition available.

Name:	fnm_Loans30To59DaysPastDuePercentageOfUnpaidPrincipalBalance
Namespace Prefix:	fnm_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

Represents the percentage unpaid principal balance comprised of single-family loans greater than or equal to 90 days past due or multifamily loans greater than or equal to 60 days past due. Calculated based on the aggregate unpaid principal balance for the category indicated, divided by the respective book of business.

No definition available.

Name:	fnm_LoansSeriouslyDelinquentPercentageOfUnpaidPrincipalBalance
Namespace Prefix:	fnm_
Data Type:	dtr-types:percentItemType
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Period Type:	instant

Represents the percentage of loans that were seriously delinquent greater than 60 days. The percentage is calculated based on the unpaid principal balance of loans that were seriously delinquent divided by the aggregate unpaid principal balance of loans for each category included in our guaranty book of business.

No definition available.

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

Represents the percentage of unpaid principal balance of loans, by category.

No definition available.

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

Represents the number of days past due required for a loan to be considered seriously delinquent.

No definition available.

Name:	fnm_SeriousDelinquencyDayspastdue1
Namespace Prefix:	fnm_
Data Type:	xbrli:durationItemType
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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.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 20
-Name Accounting Standards Codification
-Section 40
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481628/310-20-40-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481990/310-10-45-2

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Namespace Prefix:
Data Type:	na
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Name:	srt_ProductOrServiceAxis=fnm_OriginalLoanToValueRatioLessThanOrEqualToEightyPercentMember
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