Fair Value - Levels Hierarchy (Details) - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Assets:
Available-for-sale securities	$ 37,567	$ 375
Mortgage loans held for investment, net of allowance for loan losses	6,309	5,464
Derivative assets at fair value	499	177
Derivative assets, gross amount offset	(206)	(199)
Liabilities:
Derivative liabilities	640	152
Fannie Mae
Liabilities:
Long-term debt	235	256
Consolidated Trusts
Liabilities:
Long-term debt	14,728	15,060
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Trading securities	40,068	55,205
Available-for-sale securities	36,997	0
Derivative assets at fair value	0	0
Liabilities:
Derivative liabilities	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fannie Mae
Liabilities:
Long-term debt	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Consolidated Trusts
Liabilities:
Long-term debt	0	0
Significant Other Observable Inputs (Level 2)
Assets:
Trading securities	23,066	14,282
Available-for-sale securities	269	34
Derivative assets at fair value	658	304
Liabilities:
Derivative liabilities	3,269	1,387
Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt	137,415	104,784
Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt	3,440,818	3,701,675
Significant Unobservable Inputs (Level 3)
Assets:
Trading securities	22	27
Available-for-sale securities	301	341
Derivative assets at fair value	47	72
Liabilities:
Derivative liabilities	12	7
Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt	235	602
Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt	230	235
Recurring
Assets:
Trading securities	63,156	69,514
Available-for-sale securities	37,567	375
Mortgage loans held for investment, net of allowance for loan losses	6,309	5,464
Derivative assets at fair value	499	177
Derivative assets, gross amount offset	(206)	(199)
Total assets at fair value	107,531	75,530
Liabilities:
Long-term debt	14,963	15,316
Derivative liabilities	640	152
Derivative liabilities, gross amount offset	(2,641)	(1,242)
Total liabilities at fair value	15,603	15,468
Recurring | Fannie Mae
Liabilities:
Long-term debt	235	256
Recurring | Consolidated Trusts
Liabilities:
Long-term debt	14,728	15,060
Recurring | Mortgage-related securities
Assets:
Trading securities	23,069	14,291
Available-for-sale securities	570	375
Recurring | Non-mortgage-related securities
Assets:
Trading securities	40,087	55,223
Available-for-sale securities	36,997	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Trading securities	40,068	55,205
Available-for-sale securities	36,997	0
Mortgage loans held for investment, net of allowance for loan losses	0	0
Derivative assets at fair value	0	0
Total assets at fair value	77,065	55,205
Liabilities:
Long-term debt	0	0
Derivative liabilities	0	0
Total liabilities at fair value	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fannie Mae
Liabilities:
Long-term debt	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Consolidated Trusts
Liabilities:
Long-term debt	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mortgage-related securities
Assets:
Trading securities	0	0
Available-for-sale securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Non-mortgage-related securities
Assets:
Trading securities	40,068	55,205
Available-for-sale securities	36,997	0
Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Trading securities	23,066	14,282
Available-for-sale securities	269	34
Mortgage loans held for investment, net of allowance for loan losses	5,935	5,076
Derivative assets at fair value	658	304
Total assets at fair value	29,928	19,696
Liabilities:
Long-term debt	14,649	14,969
Derivative liabilities	3,269	1,387
Total liabilities at fair value	17,918	16,356
Recurring | Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt	0	0
Recurring | Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt	14,649	14,969
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-related securities
Assets:
Trading securities	23,047	14,264
Available-for-sale securities	269	34
Recurring | Significant Other Observable Inputs (Level 2) | Non-mortgage-related securities
Assets:
Trading securities	19	18
Available-for-sale securities	0	0
Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Trading securities	22	27
Available-for-sale securities	301	341
Mortgage loans held for investment, net of allowance for loan losses	374	388
Derivative assets at fair value	47	72
Total assets at fair value	744	828
Liabilities:
Long-term debt	314	347
Derivative liabilities	12	7
Total liabilities at fair value	326	354
Recurring | Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt	235	256
Recurring | Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt	79	91
Recurring | Significant Unobservable Inputs (Level 3) | Mortgage-related securities
Assets:
Trading securities	22	27
Available-for-sale securities	301	341
Recurring | Significant Unobservable Inputs (Level 3) | Non-mortgage-related securities
Assets:
Trading securities	0	0
Available-for-sale securities	$ 0	$ 0