Name:	fnm_Loans30To59DaysPastDuePercentageOfUnpaidPrincipalBalance
Namespace Prefix:	fnm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Represents the percentage of unpaid principal balance comprised of loans 60 to 89 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_Loans60To89DaysPastDuePercentageOfUnpaidPrincipalBalance
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 are 30 to 59 days past due. Calculated based on the number of loans in the delinquency bucket indicated, divided by the total number of loans in the respective category.

No definition available.

Name:	fnm_NumberOfLoans30To59DaysPastDuePercentage
Namespace Prefix:	fnm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Represents the percentage of loans that are 60 to 89 days past due. Calculated based on the number of loans in the delinquency bucket indicated, divided by the total number of loans in the respective category.

No definition available.

Name:	fnm_NumberOfLoans60To89DaysPastDuePercentage
Namespace Prefix:	fnm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Represents the percentage 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, included in each category. Calculated based on the number of loans in the delinquency bucket indicated, divided by the total number of loans in the respective category.

No definition available.

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

Name:	fnm_RiskCharacteristicsOfOurBookOfBusinessAxis=fnm_GuarantyBookOfBusinessMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_MortgageLoansOnRealEstateLoanTypeAxis=fnm_ConventionalMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	fnm_VintagesAxis=fnm_Year2005Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	fnm_VintagesAxis=fnm_Year2006Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	fnm_VintagesAxis=fnm_Year2007Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	fnm_VintagesAxis=fnm_Year2008Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	fnm_VintagesAxis=fnm_AllOtherVintagesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_ProductOrServiceAxis=fnm_AltAMortgageLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_StatementGeographicalAxis=stpr_CA
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_StatementGeographicalAxis=stpr_FL
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_StatementGeographicalAxis=stpr_IL
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_StatementGeographicalAxis=stpr_NJ
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_StatementGeographicalAxis=stpr_NY
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_StatementGeographicalAxis=fnm_OtherStatesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	fnm_EstimatedMarkToMarketLoanToValueRatioAxis=fnm_EstimatedMarkToMarketLoanToValueRatioGreaterThanOneHundredPercentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: