Fair Value Option (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at fair value	$ 6,490	$ 6,490	$ 7,825
Fair value of nonaccrual loans	139	139	129
Difference between unpaid principal balance and the fair value of nonaccrual loans	8	8	11
Fair value of loans that are 90 days past due	257	257	80
Difference between unpaid principal balance and the fair value of these 90 days or more days past due loans	14	$ 14	10
FNM_SingleFamily \| Minimum [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Serious delinquency: days past due		90 days
Fannie Mae [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Long-term debt, fair value	3,728	$ 3,728	5,687
Consolidated Trusts [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Long-term debt, fair value	24,586	24,586	21,880
Loans [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Mortgage loans held for investment, at fair value	6,490	6,490	7,825
Loans, unpaid principal balance	6,046	6,046	7,514
Fair Value, Option, Changes in Fair Value, Gain (Loss)	263		357	$ (128)
Long-term debt [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Fair Value, Option, Changes in Fair Value, Gain (Loss)		(432)	(765)	$ 688
Long-term debt [Member] \| Fannie Mae [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Long-term debt, fair value	3,728	3,728	5,687
Long-Term Debt, unpaid principal balance	3,518	3,518	5,200
Long-term debt [Member] \| Consolidated Trusts [Member]
Fair Value Inputs, Quantitative Information [Line Items]
Long-term debt, fair value	24,586	24,586	21,880
Long-Term Debt, unpaid principal balance	$ 21,408	$ 21,408	$ 19,653

Represents principal amount outstanding of loans held for investment.

No definition available.

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- Definition

Represents the number of days past due required for a loan to be considered seriously delinquent.

+ References

No definition available.

+ Details

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- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
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- Definition

For each line item in the statement of financial position, the amounts of gains and losses from fair value changes included in earnings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 30
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- Definition

This item represents the aggregate fair value of loans held as assets that are 90 days or more past due for which the fair value option has been elected.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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- Definition

For loans held as assets for which the fair value option has been elected, this item represents the difference between the aggregate fair value and the aggregate unpaid principal balance of those loans that are 90 days or more past due.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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- Definition

This item represents the aggregate fair value of loans held as assets that are in nonaccrual status for which the fair value option has been elected, if the entity's policy is to recognize interest income separately from other changes in fair value.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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- Definition

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

+ References

No definition available.

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- Definition

Fair value portion of loan receivable, including, but not limited to, mortgage loans held for investment, finance receivables held for investment, policy loans on insurance contracts.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-URI http://asc.fasb.org/subtopic&trid=2196772

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- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

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