Fair Value - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Fair Value Measurement [Domain]
Financial assets:
Derivative assets, gross amount offset	$ (233)	$ (283)
Financial liabilities:
Derivative liabilities, gross amount offset	(2,154)	(3,200)
Available-for-sale securities	492	564
Mortgage loans held for investment, net of allowance for loan losses	3,255	3,315
Derivative assets at fair value	280	202
Derivative assets, gross amount offset	(233)	(283)
Derivative liabilities at fair value	132	140
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash and cash equivalents, including restricted cash and cash equivalents	37,922	33,981
Securities purchased under agreements to resell	0	0
Trading securities	60,080	47,764
Available-for-sale securities	0	0
Mortgage loans held for sale	0	0
Mortgage loans held for investment, net of allowance for loan losses	0	0
Advances to lenders	0	0
Derivative assets at fair value	0	0
Guaranty assets and buy-ups	0	0
Total financial assets	98,002	81,745
Financial liabilities:
Derivative liabilities at fair value	0	0
Guaranty obligations	0	0
Total financial liabilities	0	0
Significant Other Observable Inputs (Level 2)
Financial assets:
Cash and cash equivalents, including restricted cash and cash equivalents	38,850	34,725
Securities purchased under agreements to resell	18,065	30,700
Trading securities	1,183	4,762
Available-for-sale securities	44	50
Mortgage loans held for sale	565	93
Mortgage loans held for investment, net of allowance for loan losses	3,645,412	3,571,555
Advances to lenders	2,595	1,389
Derivative assets at fair value	422	395
Guaranty assets and buy-ups	0	0
Total financial assets	3,707,136	3,643,669
Financial liabilities:
Derivative liabilities at fair value	2,275	3,327
Guaranty obligations	0	0
Total financial liabilities	3,838,578	3,760,502
Significant Unobservable Inputs (Level 3)
Financial assets:
Cash and cash equivalents, including restricted cash and cash equivalents	0	0
Securities purchased under agreements to resell	0	0
Trading securities	35	26
Available-for-sale securities	448	514
Mortgage loans held for sale	791	2,196
Mortgage loans held for investment, net of allowance for loan losses	131,457	130,022
Advances to lenders	0	0
Derivative assets at fair value	91	90
Guaranty assets and buy-ups	170	155
Total financial assets	132,992	133,003
Financial liabilities:
Derivative liabilities at fair value	11	13
Guaranty obligations	61	65
Total financial liabilities	980	976
Fannie Mae
Financial liabilities:
Long-term debt:	451	761
Fannie Mae \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial liabilities:
Short-term debt:	0	0
Long-term debt:	0	0
Fannie Mae \| Significant Other Observable Inputs (Level 2)
Financial liabilities:
Short-term debt:	11,424	17,317
Long-term debt:	111,915	106,701
Fannie Mae \| Significant Unobservable Inputs (Level 3)
Financial liabilities:
Short-term debt:	0	0
Long-term debt:	630	605
Consolidated Trusts
Financial liabilities:
Long-term debt:	13,237	14,343
Consolidated Trusts \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial liabilities:
Long-term debt:	0	0
Consolidated Trusts \| Significant Other Observable Inputs (Level 2)
Financial liabilities:
Long-term debt:	3,712,964	3,633,157
Consolidated Trusts \| Significant Unobservable Inputs (Level 3)
Financial liabilities:
Long-term debt:	278	293
Carrying Value
Financial assets:
Cash and cash equivalents, including restricted cash and cash equivalents	76,772	68,706
Securities purchased under agreements to resell	18,065	30,700
Trading securities	61,298	52,552
Available-for-sale securities	492	564
Mortgage loans held for sale	1,278	2,149
Mortgage loans held for investment, net of allowance for loan losses	4,137,380	4,133,482
Advances to lenders	2,595	1,389
Derivative assets at fair value	280	202
Guaranty assets and buy-ups	66	73
Total financial assets	4,298,226	4,289,817
Financial liabilities:
Derivative liabilities at fair value	132	140
Guaranty obligations	72	79
Total financial liabilities	4,217,982	4,222,937
Carrying Value \| Fannie Mae
Financial liabilities:
Short-term debt:	11,419	17,314
Long-term debt:	110,296	106,751
Carrying Value \| Consolidated Trusts
Financial liabilities:
Long-term debt:	4,096,063	4,098,653
Estimated Fair Value
Financial assets:
Cash and cash equivalents, including restricted cash and cash equivalents	76,772	68,706
Securities purchased under agreements to resell	18,065	30,700
Trading securities	61,298	52,552
Available-for-sale securities	492	564
Mortgage loans held for sale	1,356	2,289
Mortgage loans held for investment, net of allowance for loan losses	3,776,869	3,701,577
Advances to lenders	2,595	1,389
Derivative assets at fair value	280	202
Guaranty assets and buy-ups	170	155
Total financial assets	3,937,897	3,858,134
Financial liabilities:
Derivative liabilities at fair value	132	140
Guaranty obligations	61	65
Total financial liabilities	3,837,404	3,758,278
Estimated Fair Value \| Fannie Mae
Financial liabilities:
Short-term debt:	11,424	17,317
Long-term debt:	112,545	107,306
Estimated Fair Value \| Consolidated Trusts
Financial liabilities:
Long-term debt:	$ 3,713,242	$ 3,633,450

This element represents the portion of the balance sheet assertion valued at fair value by the entity 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. This item includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Company may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. The carrying amounts of cash and cash equivalent items which are restricted as to withdrawal or usage. Restrictions may include legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or entity statements of intention with regard to particular deposits; however, time deposits and short-term certificates of deposit are not generally included in legally restricted deposits. Excludes compensating balance arrangements that are not agreements which legally restrict the use of cash amounts shown on the balance sheet. This element is for unclassified presentations; for classified presentations there is a separate and distinct element.

No definition available.