Fair Value - Levels Hierarchy (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Assets:
Available-for-sale securities:	$ 445	$ 469
Mortgage loans held for investment, net of allowance for loan losses	4,508	3,744
Derivative assets at fair value	127	179
Derivative assets, gross amount offset	(399)	(362)
Liabilities:
Derivative liabilities	157	154
Fannie Mae
Liabilities:
Long-term debt:	358	385
Consolidated Trusts
Liabilities:
Long-term debt:	14,977	13,292
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Trading securities	77,865	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	115,937	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	1,011	1,090
Available-for-sale securities:	40	42
Mortgage loans held for investment, net of allowance for loan losses	3,562,659	3,496,803
Derivative assets at fair value	472	487
Total financial assets	3,638,009	3,556,627
Liabilities:
Derivative liabilities	2,031	2,506
Total financial liabilities	3,775,978	3,700,088
Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt:	127,301	129,152
Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt:	3,635,604	3,557,237
Significant Unobservable Inputs (Level 3)
Assets:
Trading securities	26	28
Available-for-sale securities:	405	427
Mortgage loans held for investment, net of allowance for loan losses	132,222	127,763
Derivative assets at fair value	54	54
Total financial assets	133,177	128,690
Liabilities:
Derivative liabilities	19	15
Total financial liabilities	934	916
Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt:	592	567
Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt:	265	274
Recurring
Assets:
Trading securities	78,902	78,728
Available-for-sale securities:	445	469
Mortgage loans held for investment, net of allowance for loan losses	4,508	3,744
Derivative assets at fair value	127	179
Derivative assets, gross amount offset	(399)	(362)
Total financial assets	83,982	83,120
Liabilities:
Long-term debt:	15,335	13,677
Derivative liabilities	157	154
Derivative liabilities, gross amount offset	(1,893)	(2,367)
Total financial liabilities	15,492	13,831
Recurring | Fannie Mae
Liabilities:
Long-term debt:	358	385
Recurring | Consolidated Trusts
Liabilities:
Long-term debt:	14,977	13,292
Recurring | Mortgage-related securities
Assets:
Trading securities	1,018	1,098
Recurring | Non-mortgage-related securities
Assets:
Trading securities	77,884	77,630
Recurring | Agency
Assets:
Available-for-sale securities:	326	339
Recurring | Other mortgage-related
Assets:
Available-for-sale securities:	119	130
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Trading securities	77,865	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	77,865	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 securities
Assets:
Trading securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Non-mortgage-related securities
Assets:
Trading securities	77,865	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	1,011	1,090
Available-for-sale securities:	40	42
Mortgage loans held for investment, net of allowance for loan losses	4,120	3,345
Derivative assets at fair value	472	487
Total financial assets	5,643	4,964
Liabilities:
Long-term debt:	14,971	13,324
Derivative liabilities	2,031	2,506
Total financial liabilities	17,002	15,830
Recurring | Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt:	95	136
Recurring | Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt:	14,876	13,188
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-related securities
Assets:
Trading securities	992	1,070
Recurring | Significant Other Observable Inputs (Level 2) | Non-mortgage-related securities
Assets:
Trading securities	19	20
Recurring | Significant Other Observable Inputs (Level 2) | Agency
Assets:
Available-for-sale securities:	36	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:	405	427
Mortgage loans held for investment, net of allowance for loan losses	388	399
Derivative assets at fair value	54	54
Total financial assets	873	908
Liabilities:
Long-term debt:	364	353
Derivative liabilities	19	15
Total financial liabilities	383	368
Recurring | Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt:	263	249
Recurring | Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt:	101	104
Recurring | Significant Unobservable Inputs (Level 3) | Mortgage-related securities
Assets:
Trading securities	26	28
Recurring | Significant Unobservable Inputs (Level 3) | Non-mortgage-related securities
Assets:
Trading securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Agency
Assets:
Available-for-sale securities:	290	301
Recurring | Significant Unobservable Inputs (Level 3) | Other mortgage-related
Assets:
Available-for-sale securities:	$ 115	$ 126