Mortgage Loans Narratives (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Mortgage Loans on Real Estate [Abstract]
Carrying value of loans redesignated from HFS to HFI	$ 285,000,000
Carrying value of loans redesignated from HFI to HFS	2,200,000,000	1,300,000,000	33,000,000
Average term extension of a single-family modified loan	161 months	154 months
Average interest rate reduction of a single-family modified loan	0.99%	1.68%
Sale of loans, unpaid principal balance	$ 1,900,000,000	$ 1,200,000,000

The following represents the average interest rate reduction (disclosed as a %) resulting from loan modifications.

No definition available.

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Namespace Prefix:	fnm_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The following represents the average term extension (disclosed in months) resulting from loan modifications.

+ References

No definition available.

+ Details

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

- Definition

The amount of financing receivable to held-for-investment during the reporting period.

+ References

No definition available.

+ Details

Name:	fnm_FinancingReceivableReclassificationtoHeldforinvestment
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of financing receivables reclassified to held for sale during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 11B

-Subparagraph (f)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953423-111524

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The amount of any significant sales of financing receivables during the reporting period.

+ References

+ Details

Name:	us-gaap_FinancingReceivableSignificantSales
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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