Derivative Instruments Derivatives 1 - Notional and FV Position (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Asset Derivatives [Abstract]
Notional amount	$ 243,475	$ 266,935
Derivative asset, estimated fair value	271	458
Netting adjustment	(1,288)	(2,266)
Derivative Liabilities [Abstract]
Notional amount	196,666	161,488
Derivative liability, estimated fair value	(435)	(777)
Netting adjustment	1,694	2,315
Table Footnote [Abstract]
Cash collateral posted for derivative instruments	1,000	713
Cash collateral received for derivative instruments	635	664
Pay-fixed swap
Asset Derivatives [Abstract]
Notional amount	41,052	71,416
Derivative asset, estimated fair value	0	438
Derivative Liabilities [Abstract]
Notional amount	29,178	21,253
Derivative liability, estimated fair value	(970)	(740)
Receive-fixed Swap
Asset Derivatives [Abstract]
Notional amount	73,579	88,799
Derivative asset, estimated fair value	816	1,113
Derivative Liabilities [Abstract]
Notional amount	26,382	58,399
Derivative liability, estimated fair value	(62)	(860)
Basis swap
Asset Derivatives [Abstract]
Notional amount	273	250
Derivative asset, estimated fair value	149	104
Derivative Liabilities [Abstract]
Notional amount	0	624
Derivative liability, estimated fair value	0	0
Foreign currency swap
Asset Derivatives [Abstract]
Notional amount	229	221
Derivative asset, estimated fair value	39	22
Derivative Liabilities [Abstract]
Notional amount	232	223
Derivative liability, estimated fair value	(65)	(72)
Pay-fixed swaption
Asset Derivatives [Abstract]
Notional amount	4,600	10,375
Derivative asset, estimated fair value	18	191
Derivative Liabilities [Abstract]
Notional amount	6,375	1,000
Derivative liability, estimated fair value	(219)	(4)
Receive-fixed swaption
Asset Derivatives [Abstract]
Notional amount	2,875	500
Derivative asset, estimated fair value	106	20
Derivative Liabilities [Abstract]
Notional amount	4,600	7,375
Derivative liability, estimated fair value	(232)	(338)
Futures
Asset Derivatives [Abstract]
Notional amount	20,507	16,631
Derivative asset, estimated fair value	0	0
Derivative Liabilities [Abstract]
Notional amount	0	0
Derivative liability, estimated fair value	0	0
Risk management derivatives
Asset Derivatives [Abstract]
Notional amount	143,115	188,192
Derivative asset, estimated fair value	1,128	1,888
Accrued interest receivable	226	400
Netting adjustment	(1,288)	(2,266)
Total net risk management derivatives - Asset	66	22
Derivative Liabilities [Abstract]
Notional amount	66,767	88,874
Derivative liability, estimated fair value	(1,548)	(2,014)
Accrued interest payable	(250)	(419)
Netting adjustment	1,694	2,315
Total net risk management derivatives - Liability	(104)	(118)
Mortgage commitments to purchase whole loans
Asset Derivatives [Abstract]
Notional amount	7,115	4,370
Derivative asset, estimated fair value	15	29
Derivative Liabilities [Abstract]
Notional amount	1,787	57
Derivative liability, estimated fair value	(1)	0
Forward contracts to purchase mortgage-related securities
Asset Derivatives [Abstract]
Notional amount	55,531	40,650
Derivative asset, estimated fair value	137	349
Derivative Liabilities [Abstract]
Notional amount	9,560	1,045
Derivative liability, estimated fair value	(28)	(3)
Forward contracts to sell mortgage-related securities
Asset Derivatives [Abstract]
Notional amount	9,282	292
Derivative asset, estimated fair value	13	1
Derivative Liabilities [Abstract]
Notional amount	109,066	70,593
Derivative liability, estimated fair value	(277)	(645)
Mortgage commitment derivatives
Asset Derivatives [Abstract]
Notional amount	71,928	45,312
Derivative asset, estimated fair value	165	379
Netting adjustment	0	0
Derivative Liabilities [Abstract]
Notional amount	120,413	71,695
Derivative liability, estimated fair value	(306)	(648)
Netting adjustment	0	0
Credit enhancement derivatives
Asset Derivatives [Abstract]
Notional amount	28,432	33,431
Derivative asset, estimated fair value	40	57
Derivative Liabilities [Abstract]
Notional amount	9,486	919
Derivative liability, estimated fair value	$ (25)	$ (11)

Represents carrying amount of interest earned on derivative assets but not received, as of the balance sheet date.

No definition available.

Name:	fnm_DerivativeAssetsInterestReceivable
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of derivative asset, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position. Includes all derivative assets subject to and not subject to master netting arrangements.

+ References

No definition available.

+ Details

Name:	fnm_DerivativeFairValueGrossAssetAllDerivatives
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value of derivative liability, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position. Includes all derivatives liability subject and not subject to master netting arrangements.

+ References

No definition available.

+ Details

Name:	fnm_DerivativeLiabilityFairValueGrossLiabilityAllDerivatives
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents carrying value of accrued interest payable on derivative liabilities that has been incurred and is unpaid, as of the balance sheet date.

+ References

No definition available.

+ Details

Name:	fnm_DerivativeLiabilityInterestPayable
Namespace Prefix:	fnm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Represents a tabular schedule for the footnotes to a table.

+ References

No definition available.

+ Details

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

- Definition

Amount of obligation to return cash collateral under master netting arrangements offset against derivative assets.

+ References

+ Details

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

- Definition

Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624163-113959

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226003-175313

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

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

- Definition

Nominal or face amount used to calculate payments on the derivative asset.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeAssetNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=118260190&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

+ Details

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

- References

No definition available.

+ Details

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

- Definition

Amount of right to receive cash collateral under master netting arrangements offset against derivative liabilities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=117331979&loc=d3e41228-113958

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

+ Details

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

- Definition

Fair value of asset associated with financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226003-175313

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
-Subparagraph (a),(c)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624163-113959

+ Details

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

- Definition

Nominal or face amount used to calculate payments on the derivative liability.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

+ Details

Name:	us-gaap_DerivativeLiabilityNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fnm_PayFixedSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fnm_ReceiveFixedSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_BasisSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_CurrencySwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fnm_PayFixedSwaptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fnm_ReceiveFixedSwaptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_FutureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fnm_MortgageCommitmentsToPurchaseWholeLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fnm_ForwardContractsToPurchaseMortgageRelatedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fnm_ForwardContractsToSellMortgageRelatedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fnm_MortgageCommitmentDerivativesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_OtherContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: