Fair Value - Recurring Changes in Fair Value (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Assets:
Available-for-sale securities	$ 469	$ 564
Total multifamily mortgage loans held for investment, at amortized cost	$ 3,744	$ 3,315
Derivative Asset Statement Of Financial Position Extensible Enumeration Not Disclosed Flag	Derivative assets	Derivative assets
Derivative assets at fair value	$ 179	$ 202
Derivative asset, gross amount offset	$ (362)	$ (283)
Liabilities:
Derivative Liability Statement Of Financial Position Extensible Enumeration Not Disclosed Flag	Derivative liabilities	Derivative liabilities
Derivative liabilities at fair value	$ 154	$ 140
Fannie Mae
Liabilities:
Long-term debt	385	761
Consolidated Trusts
Liabilities:
Long-term debt	13,292	14,343
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Trading securities	77,610	47,764
Available-for-sale securities	0	0
Total multifamily mortgage loans held for investment, at amortized cost	0	0
Derivative assets at fair value	0	0
Total assets at fair value	116,146	81,745
Liabilities:
Derivative liabilities at fair value	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	1,090	4,762
Available-for-sale securities	42	50
Total multifamily mortgage loans held for investment, at amortized cost	3,496,803	3,571,555
Derivative assets at fair value	487	395
Total assets at fair value	3,556,627	3,643,669
Liabilities:
Derivative liabilities at fair value	2,506	3,327
Total financial liabilities	3,700,088	3,760,502
Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt	129,152	106,701
Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt	3,557,237	3,633,157
Significant Unobservable Inputs (Level 3)
Assets:
Trading securities	28	26
Available-for-sale securities	427	514
Total multifamily mortgage loans held for investment, at amortized cost	127,763	130,022
Derivative assets at fair value	54	90
Total assets at fair value	128,690	133,003
Liabilities:
Derivative liabilities at fair value	15	13
Total financial liabilities	916	976
Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt	567	605
Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt	274	293
Recurring
Assets:
Trading securities	78,728	52,552
Available-for-sale securities	469	564
Total multifamily mortgage loans held for investment, at amortized cost	3,744	3,315
Derivative assets at fair value	179	202
Derivative asset, gross amount offset	(362)	(283)
Total assets at fair value	83,120	56,633
Liabilities:
Long-term debt	13,677	15,104
Derivative liabilities at fair value	154	140
Derivative liabilities, gross amount offset	(2,367)	(3,200)
Total financial liabilities	13,831	15,244
Recurring | Mortgage-related securities
Assets:
Trading securities	1,098	4,770
Recurring | Non-mortgage-related securities
Assets:
Trading securities	77,630	47,782
Recurring | Agency
Assets:
Available-for-sale securities	339	377
Recurring | Other
Assets:
Available-for-sale securities	130	187
Recurring | Fannie Mae
Liabilities:
Long-term debt	385	761
Recurring | Consolidated Trusts
Liabilities:
Long-term debt	13,292	14,343
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets:
Trading securities	77,610	47,764
Available-for-sale securities	0	0
Total multifamily mortgage loans held for investment, at amortized cost	0	0
Derivative assets at fair value	0	0
Total assets at fair value	77,610	47,764
Liabilities:
Long-term debt	0	0
Derivative liabilities at fair value	0	0
Total financial liabilities	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	77,610	47,764
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
Assets:
Available-for-sale securities	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 | Significant Other Observable Inputs (Level 2)
Assets:
Trading securities	1,090	4,762
Available-for-sale securities	42	50
Total multifamily mortgage loans held for investment, at amortized cost	3,345	2,838
Derivative assets at fair value	487	395
Total assets at fair value	4,964	8,045
Liabilities:
Long-term debt	13,324	14,719
Derivative liabilities at fair value	2,506	3,327
Total financial liabilities	15,830	18,046
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage-related securities
Assets:
Trading securities	1,070	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:
Available-for-sale securities	38	46
Recurring | Significant Other Observable Inputs (Level 2) | Other
Assets:
Available-for-sale securities	4	4
Recurring | Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities:
Long-term debt	136	493
Recurring | Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities:
Long-term debt	13,188	14,226
Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Trading securities	28	26
Available-for-sale securities	427	514
Total multifamily mortgage loans held for investment, at amortized cost	399	477
Derivative assets at fair value	54	90
Total assets at fair value	908	1,107
Liabilities:
Long-term debt	353	385
Derivative liabilities at fair value	15	13
Total financial liabilities	368	398
Recurring | Significant Unobservable Inputs (Level 3) | Mortgage-related securities
Assets:
Trading securities	28	26
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	301	331
Recurring | Significant Unobservable Inputs (Level 3) | Other
Assets:
Available-for-sale securities	126	183
Recurring | Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities:
Long-term debt	249	268
Recurring | Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities:
Long-term debt	$ 104	$ 117