Name:	fnm_CurrentDebtServiceCoverageRatioHigherRiskLoans1
Namespace Prefix:	fnm_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

Our estimate of current debt service coverage ratios are based on the latest available income information for these properties. Although we use the most recently available results of our multifamily borrowers, there is a lag in reporting, which ranges, as they prepare their results in the normal course of business.

No definition available.

Name:	fnm_CurrentDebtservicecoverageratioreportinglag1
Namespace Prefix:	fnm_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

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:	num: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:	num:percentItemType
Balance Type:	na
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:	num:percentItemType
Balance Type:	na
Period Type:	instant

Represents the percentage of loans with detailed loan level information available.

No definition available.

Name:	fnm_PercentageOfLoansWithDetailedLoanLevelInformation
Namespace Prefix:	fnm_
Data Type:	num: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:	num: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
Balance Type:	na
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_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Name:	srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis=srt_MultifamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_ConcentrationRiskByBenchmarkAxis=fnm_GuarantyBookOfBusinessMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	srt_ProductOrServiceAxis=fnm_OriginalLoanToValueRatioGreaterThanEightyPercentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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