Fair Value - Levels Hierarchy (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Assets:
Total Fair Value	$ 507	$ 564
Mortgage loans held for investment, net of allowance for loan losses	3,194	3,315
Derivative assets at fair value	219	202
Derivative assets, gross amount offset	(290)	(283)
Liabilities:
Derivative liabilities	81	140
Fannie Mae
Liabilities:
Long-term debt:	544	761
Consolidated Trusts
Liabilities:
Long-term debt:	13,239	14,343
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Trading securities	48,045	47,764
Total Fair Value	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	83,808	81,745
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	1,315	4,762
Total Fair Value	46	50
Mortgage loans held for investment, net of allowance for loan losses	3,493,410	3,571,555
Derivative assets at fair value	400	395
Total financial assets	3,567,306	3,643,669
Liabilities:
Derivative liabilities	3,092	3,327
Total financial liabilities	3,675,544	3,760,502
Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt:	106,618	106,701
Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt:	3,553,827	3,633,157
Significant Unobservable Inputs (Level 3)
Assets:
Trading securities	32	26
Total Fair Value	461	514
Mortgage loans held for investment, net of allowance for loan losses	127,303	130,022
Derivative assets at fair value	109	90
Total financial assets	128,674	133,003
Liabilities:
Derivative liabilities	15	13
Total financial liabilities	984	976
Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt:	631	605
Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt:	279	293
Recurring
Assets:
Trading securities	49,392	52,552
Total Fair Value	507	564
Mortgage loans held for investment, net of allowance for loan losses	3,194	3,315
Derivative assets at fair value	219	202
Derivative assets, gross amount offset	(290)	(283)
Total financial assets	53,312	56,633
Liabilities:
Long-term debt:	13,783	15,104
Derivative liabilities	81	140
Derivative liabilities, gross amount offset	(3,026)	(3,200)
Total financial liabilities	13,864	15,244
Recurring | Fannie Mae
Liabilities:
Long-term debt:	544	761
Recurring | Consolidated Trusts
Liabilities:
Long-term debt:	13,239	14,343
Recurring | Mortgage-related
Assets:
Trading securities	1,327	4,770
Recurring | Non-mortgage-related securities
Assets:
Trading securities	48,065	47,782
Recurring | Agency
Assets:
Total Fair Value	356	377
Recurring | Other mortgage-related
Assets:
Total Fair Value	151	187
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Trading securities	48,045	47,764
Total Fair Value	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	48,045	47,764
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 securities
Assets:
Trading securities	48,045	47,764
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Agency
Assets:
Total Fair Value	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other mortgage-related
Assets:
Total Fair Value	0	0
Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Trading securities	1,315	4,762
Total Fair Value	46	50
Mortgage loans held for investment, net of allowance for loan losses	2,769	2,838
Derivative assets at fair value	400	395
Total financial assets	4,530	8,045
Liabilities:
Long-term debt:	13,403	14,719
Derivative liabilities	3,092	3,327
Total financial liabilities	16,495	18,046
Recurring | Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt:	274	493
Recurring | Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt:	13,129	14,226
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-related
Assets:
Trading securities	1,295	4,744
Recurring | Significant Other Observable Inputs (Level 2) | Non-mortgage-related securities
Assets:
Trading securities	20	18
Recurring | Significant Other Observable Inputs (Level 2) | Agency
Assets:
Total Fair Value	41	46
Recurring | Significant Other Observable Inputs (Level 2) | Other mortgage-related
Assets:
Total Fair Value	5	4
Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Trading securities	32	26
Total Fair Value	461	514
Mortgage loans held for investment, net of allowance for loan losses	425	477
Derivative assets at fair value	109	90
Total financial assets	1,027	1,107
Liabilities:
Long-term debt:	380	385
Derivative liabilities	15	13
Total financial liabilities	395	398
Recurring | Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt:	270	268
Recurring | Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt:	110	117
Recurring | Significant Unobservable Inputs (Level 3) | Mortgage-related
Assets:
Trading securities	32	26
Recurring | Significant Unobservable Inputs (Level 3) | Non-mortgage-related securities
Assets:
Trading securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Agency
Assets:
Total Fair Value	315	331
Recurring | Significant Unobservable Inputs (Level 3) | Other mortgage-related
Assets:
Total Fair Value	$ 146	$ 183