Fair Value - Levels Hierarchy (Details) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Assets:
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)
Liabilities:
Derivative liabilities	179	154
Fannie Mae
Liabilities:
Long-term debt:	299	385
Consolidated Trusts
Liabilities:
Long-term debt:	15,323	13,292
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Trading securities	66,149	77,610
Available-for-sale securities:	0	0
Mortgage loans held for investment, net of allowance for loan losses	0	0
Derivative assets at fair value	0	0
Total financial assets	105,524	116,146
Liabilities:
Derivative liabilities	0	0
Total financial 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	5,093	1,090
Available-for-sale securities:	37	42
Mortgage loans held for investment, net of allowance for loan losses	3,640,183	3,496,803
Derivative assets at fair value	364	487
Total financial assets	3,710,950	3,556,627
Liabilities:
Derivative liabilities	1,494	2,506
Total financial liabilities	3,826,156	3,700,088
Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt:	108,474	129,152
Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt:	3,696,260	3,557,237
Significant Unobservable Inputs (Level 3)
Assets:
Trading securities	26	28
Available-for-sale securities:	351	427
Mortgage loans held for investment, net of allowance for loan losses	134,684	127,763
Derivative assets at fair value	57	54
Total financial assets	135,600	128,690
Liabilities:
Derivative liabilities	9	15
Total financial liabilities	908	916
Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt:	601	567
Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt:	247	274
Recurring
Assets:
Trading securities	71,268	78,728
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)
Total financial assets	77,002	83,120
Liabilities:
Long-term debt:	15,622	13,677
Derivative liabilities	179	154
Derivative liabilities, gross amount offset	(1,324)	(2,367)
Total financial liabilities	15,801	13,831
Recurring | Fannie Mae
Liabilities:
Long-term debt:	299	385
Recurring | Consolidated Trusts
Liabilities:
Long-term debt:	15,323	13,292
Recurring | Mortgage-related
Assets:
Trading securities	5,102	1,098
Recurring | Non-mortgage-related
Assets:
Trading securities	66,166	77,630
Recurring | Agency
Assets:
Available-for-sale securities:	316	339
Recurring | Other mortgage-related
Assets:
Available-for-sale securities:	72	130
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Trading securities	66,149	77,610
Available-for-sale securities:	0	0
Mortgage loans held for investment, net of allowance for loan losses	0	0
Derivative assets at fair value	0	0
Total financial assets	66,149	77,610
Liabilities:
Long-term debt:	0	0
Derivative liabilities	0	0
Total financial liabilities	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
Assets:
Trading securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Non-mortgage-related
Assets:
Trading securities	66,149	77,610
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Agency
Assets:
Available-for-sale securities:	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other mortgage-related
Assets:
Available-for-sale securities:	0	0
Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Trading securities	5,093	1,090
Available-for-sale securities:	37	42
Mortgage loans held for investment, net of allowance for loan losses	4,831	3,345
Derivative assets at fair value	364	487
Total financial assets	10,325	4,964
Liabilities:
Long-term debt:	15,270	13,324
Derivative liabilities	1,494	2,506
Total financial liabilities	16,764	15,830
Recurring | Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt:	41	136
Recurring | Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt:	15,229	13,188
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-related
Assets:
Trading securities	5,076	1,070
Recurring | Significant Other Observable Inputs (Level 2) | Non-mortgage-related
Assets:
Trading securities	17	20
Recurring | Significant Other Observable Inputs (Level 2) | Agency
Assets:
Available-for-sale securities:	33	38
Recurring | Significant Other Observable Inputs (Level 2) | Other mortgage-related
Assets:
Available-for-sale securities:	4	4
Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Trading securities	26	28
Available-for-sale securities:	351	427
Mortgage loans held for investment, net of allowance for loan losses	371	399
Derivative assets at fair value	57	54
Total financial assets	805	908
Liabilities:
Long-term debt:	352	353
Derivative liabilities	9	15
Total financial liabilities	361	368
Recurring | Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt:	258	249
Recurring | Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt:	94	104
Recurring | Significant Unobservable Inputs (Level 3) | Mortgage-related
Assets:
Trading securities	26	28
Recurring | Significant Unobservable Inputs (Level 3) | Non-mortgage-related
Assets:
Trading securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Agency
Assets:
Available-for-sale securities:	283	301
Recurring | Significant Unobservable Inputs (Level 3) | Other mortgage-related
Assets:
Available-for-sale securities:	$ 68	$ 126