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

Line items represent the UPB and modeled attributes included in a table. These are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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

Represents the expected remaining cumulative default rate of the collateral pool backing the securities, as a percentage of the current collateral unpaid principal balance, weighted by security unpaid principal balance.

No definition available.

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

Represents the expected remaining loss given default of the collateral pool backing the securities, calculated as the ratio of remaining cumulative loss divided by cumulative defaults, weighted by security unpaid principal balance.

No definition available.

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

Represents the average monthly voluntary prepayment rate, weighted by security unpaid principal balance.

No definition available.

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

For investments which are quantified by principal amount, the principle balance held at close of period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 6
-Subparagraph (SX 210.12-14.Column B)
-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611322-123010

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-12.Column B)
-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611133-123010

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 946
-SubTopic 320
-Section S99
-Paragraph 4
-Subparagraph (SX 210.12-12C.Column B)
-URI http://asc.fasb.org/extlink&oid=50485924&loc=d3e611225-123010

Name:	us-gaap_InvestmentOwnedBalancePrincipalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- References

No definition available.

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

This element represents the amount of loss recognized for other than temporary impairments (OTTI) of investments in debt and equity securities.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 5.M)
-URI http://asc.fasb.org/extlink&oid=27011274&loc=d3e87990-122713

Name:	us-gaap_MarketableSecuritiesRealizedGainLossOtherThanTemporaryImpairmentsAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=fnm_MortgageBackedSecuritiesIssuedByPrivateEnterprisesSubprimeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=fnm_AltAAdjustableRateMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=fnm_AltAFixedRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis=fnm_AltAVariableRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: