Fair Value - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Fair Value Measurement [Domain]
Financial assets:
Derivative assets, gross amount offset	$ (206)	$ (199)
Financial liabilities:
Derivative liabilities, gross amount offset	(2,641)	(1,242)
Available-for-sale securities	37,567	375
Derivative assets at fair value	499	177
Derivative assets, gross amount offset	(206)	(199)
Derivative liabilities at fair value	640	152
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash, including restricted cash	36,343	42,583
Securities purchased under agreements to resell	0	0
Trading securities	40,068	55,205
Available-for-sale securities	36,997	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	0	0
Total financial assets	113,408	97,788
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, including restricted cash	0	0
Securities purchased under agreements to resell	33,450	45,650
Trading securities	23,066	14,282
Available-for-sale securities	269	34
Mortgage loans held for sale	215	23
Mortgage loans held for investment, net of allowance for loan losses	3,437,626	3,656,465
Advances to lenders	1,206	3,595
Derivative assets at fair value	658	304
Guaranty assets	0	0
Total financial assets	3,496,490	3,720,353
Financial liabilities:
Derivative liabilities at fair value	3,269	1,387
Guaranty obligations	0	0
Total financial liabilities	3,612,619	3,832,394
Significant Unobservable Inputs (Level 3)
Financial assets:
Cash, including restricted cash	0	0
Securities purchased under agreements to resell	0	0
Trading securities	22	27
Available-for-sale securities	301	341
Mortgage loans held for sale	150	201
Mortgage loans held for investment, net of allowance for loan losses	154,860	137,864
Advances to lenders	0	0
Derivative assets at fair value	47	72
Guaranty assets	171	172
Total financial assets	155,551	138,677
Financial liabilities:
Derivative liabilities at fair value	12	7
Guaranty obligations	51	54
Total financial liabilities	528	898
Fannie Mae
Financial liabilities:
Long-term debt	235	256
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:	31,117	24,548
Long-term debt	137,415	104,784
Fannie Mae \| Significant Unobservable Inputs (Level 3)
Financial liabilities:
Short-term debt:	0	0
Long-term debt	235	602
Consolidated Trusts
Financial liabilities:
Long-term debt	14,728	15,060
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	3,440,818	3,701,675
Consolidated Trusts \| Significant Unobservable Inputs (Level 3)
Financial liabilities:
Long-term debt	230	235
Carrying Value
Financial assets:
Cash, including restricted cash	36,343	42,583
Securities purchased under agreements to resell	33,450	45,650
Trading securities	63,156	69,514
Available-for-sale securities	37,567	375
Mortgage loans held for sale	332	209
Mortgage loans held for investment, net of allowance for loan losses	4,106,299	4,119,104
Advances to lenders	1,206	3,595
Derivative assets at fair value	499	177
Guaranty assets	95	90
Total financial assets	4,278,947	4,281,297
Financial liabilities:
Derivative liabilities at fair value	640	152
Guaranty obligations	100	96
Total financial liabilities	4,169,989	4,180,677
Carrying Value \| Fannie Mae
Financial liabilities:
Short-term debt:	31,120	24,538
Long-term debt	137,611	102,751
Carrying Value \| Consolidated Trusts
Financial liabilities:
Long-term debt	4,000,518	4,053,140
Estimated Fair Value
Financial assets:
Cash, including restricted cash	36,343	42,583
Securities purchased under agreements to resell	33,450	45,650
Trading securities	63,156	69,514
Available-for-sale securities	37,567	375
Mortgage loans held for sale	365	224
Mortgage loans held for investment, net of allowance for loan losses	3,592,486	3,794,329
Advances to lenders	1,206	3,595
Derivative assets at fair value	499	177
Guaranty assets	171	172
Total financial assets	3,765,243	3,956,619
Financial liabilities:
Derivative liabilities at fair value	640	152
Guaranty obligations	51	54
Total financial liabilities	3,610,506	3,832,050
Estimated Fair Value \| Fannie Mae
Financial liabilities:
Short-term debt:	31,117	24,548
Long-term debt	137,650	105,386
Estimated Fair Value \| Consolidated Trusts
Financial liabilities:
Long-term debt	$ 3,441,048	$ 3,701,910

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.