Mortgage Loans Individually Impaired Loans (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: unpaid principal balance	$ 177,630	$ 185,695
Individually impaired loans with related allowance recorded: total recorded investment	168,510	175,619
Related allowance for loan losses	31,375	37,729
Individually impaired loans with no related allowance recorded: unpaid principal balance	22,563	20,452
Individually impaired loans with no related allowance recorded: total recorded investment	19,676	17,660
Total individually impaired loans: unpaid principal balance	200,193	206,147
Total individually impaired loans: total recorded investment	188,186	193,279
Interest Receivable [Member]
Financing Receivable, Impaired [Line Items]
Related allowance for loan losses	501	688
Single-Family [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: unpaid principal balance	176,400	183,438
Individually impaired loans with related allowance recorded: total recorded investment	167,269	173,343
Related allowance for loan losses	31,200	37,423
Individually impaired loans with no related allowance recorded: unpaid principal balance	21,998	18,525
Individually impaired loans with no related allowance recorded: total recorded investment	19,108	15,721
Table Footnote [Abstract]
Troubled debt restructuring recorded investment	185,200	187,600	193,400
Single-Family [Member] \| Interest Receivable [Member]
Financing Receivable, Impaired [Line Items]
Related allowance for loan losses	495	678
Multifamily [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: unpaid principal balance	1,230	2,257
Individually impaired loans with related allowance recorded: total recorded investment	1,241	2,276
Related allowance for loan losses	175	306
Individually impaired loans with no related allowance recorded: unpaid principal balance	565	1,927
Individually impaired loans with no related allowance recorded: total recorded investment	568	1,939
Table Footnote [Abstract]
Troubled debt restructuring recorded investment	716	911	1,100
Multifamily [Member] \| Interest Receivable [Member]
Financing Receivable, Impaired [Line Items]
Related allowance for loan losses	6	10
Single-Family [Member] \| Primary [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: unpaid principal balance	125,960	130,080
Individually impaired loans with related allowance recorded: total recorded investment	120,221	123,631
Related allowance for loan losses	20,327	24,145
Individually impaired loans with no related allowance recorded: unpaid principal balance	16,704	14,076
Individually impaired loans with no related allowance recorded: total recorded investment	14,876	12,305
Single-Family [Member] \| Primary [Member] \| Interest Receivable [Member]
Financing Receivable, Impaired [Line Items]
Related allowance for loan losses	309	430
Single-Family [Member] \| Government [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: unpaid principal balance	281	213
Individually impaired loans with related allowance recorded: total recorded investment	285	210
Related allowance for loan losses	46	35
Individually impaired loans with no related allowance recorded: unpaid principal balance	61	120
Individually impaired loans with no related allowance recorded: total recorded investment	57	120
Single-Family [Member] \| Government [Member] \| Interest Receivable [Member]
Financing Receivable, Impaired [Line Items]
Related allowance for loan losses	12	5
Single-Family [Member] \| Alt-A [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: unpaid principal balance	35,492	37,356
Individually impaired loans with related allowance recorded: total recorded investment	32,816	34,479
Related allowance for loan losses	7,778	9,364
Individually impaired loans with no related allowance recorded: unpaid principal balance	3,993	3,290
Individually impaired loans with no related allowance recorded: total recorded investment	3,119	2,428
Single-Family [Member] \| Alt-A [Member] \| Interest Receivable [Member]
Financing Receivable, Impaired [Line Items]
Related allowance for loan losses	136	187
Single-Family [Member] \| Other [Member]
Financing Receivable, Impaired [Line Items]
Individually impaired loans with related allowance recorded: unpaid principal balance	14,667	15,789
Individually impaired loans with related allowance recorded: total recorded investment	13,947	15,023
Related allowance for loan losses	3,049	3,879
Individually impaired loans with no related allowance recorded: unpaid principal balance	1,240	1,039
Individually impaired loans with no related allowance recorded: total recorded investment	1,056	868
Single-Family [Member] \| Other [Member] \| Interest Receivable [Member]
Financing Receivable, Impaired [Line Items]
Related allowance for loan losses	$ 38	$ 56

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

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ 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 (g)

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

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

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

- Definition

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 40

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18845-111554

+ Details

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

- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 14A

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=SL6953550-111524

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 15

-Subparagraph (a)(4)

-URI http://asc.fasb.org/extlink&oid=35738203&loc=d3e5212-111524

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of unpaid principal balance of impaired financing receivables for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Definition

Unpaid principal balance of impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=fnm_InterestReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fnm_MultifamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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