Name:	fnm_CurrentDebtServiceCoverageRatioHigherRiskLoans1
Namespace Prefix:	fnm_
Data Type:	xbrli:pureItemType
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 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 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

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

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: