Fair Value - Fair Value of Financial Instruments (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Fair Value Measurement [Domain]
Financial assets:
Derivative assets, gross amount offset	$ (277)	$ (362)
Financial liabilities:
Derivative liabilities, gross amount offset	(1,324)	(2,367)
Available-for-sale securities	388	469
Mortgage loans held for investment, net of allowance for loan losses	5,202	3,744
Derivative assets at fair value	144	179
Derivative assets, gross amount offset	(277)	(362)
Derivative liabilities at fair value	179	154
Quoted Prices in Active Markets for Identical Assets (Level 1)
Financial assets:
Cash, including restricted cash	39,375	38,536
Securities purchased under agreements to resell	0	0
Trading securities	66,149	77,610
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	0	0
Total financial assets	105,524	116,146
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	61,525	56,250
Trading securities	5,093	1,090
Available-for-sale securities	37	42
Mortgage loans held for sale	521	130
Mortgage loans held for investment, net of allowance for loan losses	3,640,183	3,496,803
Advances to lenders	3,227	1,825
Derivative assets at fair value	364	487
Guaranty assets	0	0
Total financial assets	3,710,950	3,556,627
Financial liabilities:
Derivative liabilities at fair value	1,494	2,506
Guaranty obligations	0	0
Total financial liabilities	3,826,156	3,700,088
Significant Unobservable Inputs (Level 3)
Financial assets:
Cash, including restricted cash	0	0
Securities purchased under agreements to resell	0	0
Trading securities	26	28
Available-for-sale securities	351	427
Mortgage loans held for sale	338	258
Mortgage loans held for investment, net of allowance for loan losses	134,684	127,763
Advances to lenders	0	0
Derivative assets at fair value	57	54
Guaranty assets	144	160
Total financial assets	135,600	128,690
Financial liabilities:
Derivative liabilities at fair value	9	15
Guaranty obligations	51	60
Total financial liabilities	908	916
Fannie Mae
Financial liabilities:
Long-term debt:	299	385
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:	19,928	11,193
Long-term debt:	108,474	129,152
Fannie Mae \| Significant Unobservable Inputs (Level 3)
Financial liabilities:
Short-term debt:	0	0
Long-term debt:	601	567
Consolidated Trusts
Financial liabilities:
Long-term debt:	15,323	13,292
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:	3,696,260	3,557,237
Consolidated Trusts \| Significant Unobservable Inputs (Level 3)
Financial liabilities:
Long-term debt:	247	274
Carrying Value
Financial assets:
Cash, including restricted cash	39,375	38,536
Securities purchased under agreements to resell	61,525	56,250
Trading securities	71,268	78,728
Available-for-sale securities	388	469
Mortgage loans held for sale	808	373
Mortgage loans held for investment, net of allowance for loan losses	4,122,582	4,137,633
Advances to lenders	3,227	1,825
Derivative assets at fair value	144	179
Guaranty assets	57	64
Total financial assets	4,299,374	4,314,057
Financial liabilities:
Derivative liabilities at fair value	179	154
Guaranty obligations	62	69
Total financial liabilities	4,203,576	4,228,320
Carrying Value \| Fannie Mae
Financial liabilities:
Short-term debt:	19,922	11,188
Long-term debt:	106,468	128,234
Carrying Value \| Consolidated Trusts
Financial liabilities:
Long-term debt:	4,076,945	4,088,675
Estimated Fair Value
Financial assets:
Cash, including restricted cash	39,375	38,536
Securities purchased under agreements to resell	61,525	56,250
Trading securities	71,268	78,728
Available-for-sale securities	388	469
Mortgage loans held for sale	859	388
Mortgage loans held for investment, net of allowance for loan losses	3,774,867	3,624,566
Advances to lenders	3,227	1,825
Derivative assets at fair value	144	179
Guaranty assets	144	160
Total financial assets	3,951,797	3,801,101
Financial liabilities:
Derivative liabilities at fair value	179	154
Guaranty obligations	51	60
Total financial liabilities	3,825,740	3,698,637
Estimated Fair Value \| Fannie Mae
Financial liabilities:
Short-term debt:	19,928	11,193
Long-term debt:	109,075	129,719
Estimated Fair Value \| Consolidated Trusts
Financial liabilities:
Long-term debt:	$ 3,696,507	$ 3,557,511

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.