Fair Value - Levels Hierarchy (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Assets [Abstract]
Mortgage loans held for investment, net of allowance for loan losses	$ 5,616	$ 6,490
Other assets [Abstract]
Derivative assets, gross amount offset	(331)	(905)
Other liabilities [Abstract]
Derivative liabilities, gross amount offset	(849)	(995)
Mortgage commitment derivatives
Other assets [Abstract]
Derivative assets, gross amount offset	0	0
Other liabilities [Abstract]
Derivative liabilities, gross amount offset	0	0
Fannie Mae
Liabilities [Abstract]
Long-term debt at fair value	2,946	3,728
Consolidated Trusts
Liabilities [Abstract]
Long-term debt at fair value	22,972	24,586
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets [Abstract]
Trading securities	89,707	130,456
Available-for-sale securities	0	0
Mortgage loans held for investment, net of allowance for loan losses	0	0
Other assets [Abstract]
Derivative assets at fair value	0	0
Total nonrecurring assets at fair value	168,056	227,635
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Total liabilities at fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fannie Mae
Liabilities [Abstract]
Long-term debt at fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Consolidated Trusts
Liabilities [Abstract]
Long-term debt at fair value	0	0
Significant Other Observable Inputs (Level 2)
Assets [Abstract]
Trading securities	5,253	5,991
Available-for-sale securities	503	1,049
Mortgage loans held for investment, net of allowance for loan losses	3,688,808	3,512,672
Other assets [Abstract]
Derivative assets at fair value	500	1,748
Total nonrecurring assets at fair value	3,794,016	3,578,668
Other liabilities [Abstract]
Derivative liabilities at fair value	1,335	2,441
Total liabilities at fair value	4,153,252	4,059,705
Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities [Abstract]
Long-term debt at fair value	258,200	288,414
Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities [Abstract]
Long-term debt at fair value	3,891,437	3,756,673
Significant Unobservable Inputs (Level 3)
Assets [Abstract]
Trading securities	141	95
Available-for-sale securities	452	648
Mortgage loans held for investment, net of allowance for loan losses	234,637	255,556
Other assets [Abstract]
Derivative assets at fair value	226	382
Total nonrecurring assets at fair value	241,668	262,442
Other liabilities [Abstract]
Derivative liabilities at fair value	32	49
Total liabilities at fair value	31,993	32,593
Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities [Abstract]
Long-term debt at fair value	844	878
Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities [Abstract]
Long-term debt at fair value	31,043	31,584
Recurring
Assets [Abstract]
Trading securities	95,101	136,542
Available-for-sale securities	955	1,697
Mortgage loans held for investment, net of allowance for loan losses	5,616	6,490
Other assets [Abstract]
Derivative assets at fair value	395	1,225
Derivative assets, gross amount offset	(331)	(905)
Total nonrecurring assets at fair value	105,191	147,074
Liabilities [Abstract]
Long-term debt at fair value	25,918	28,314
Other liabilities [Abstract]
Derivative liabilities at fair value	518	1,495
Derivative liabilities, gross amount offset	(849)	(995)
Total liabilities at fair value	26,436	29,809
Cash and Cash Equivalents, Fair Value Disclosure	3,124	1,120
Recurring | Swaps [Member]
Other assets [Abstract]
Derivative assets at fair value	294	579
Other liabilities [Abstract]
Derivative liabilities at fair value	813	881
Recurring | Swaptions [Member]
Other assets [Abstract]
Derivative assets at fair value	71	383
Other liabilities [Abstract]
Derivative liabilities at fair value	46	134
Recurring | Mortgage commitment derivatives
Other assets [Abstract]
Derivative assets at fair value	311	989
Other liabilities [Abstract]
Derivative liabilities at fair value	476	1,426
Recurring | Other
Other assets [Abstract]
Derivative assets at fair value	50	179
Other liabilities [Abstract]
Derivative liabilities at fair value	32	49
Recurring | Fannie Mae
Liabilities [Abstract]
Long-term debt at fair value	2,946	3,728
Recurring | Fannie Mae | Total senior floating
Liabilities [Abstract]
Long-term debt at fair value	2,946	3,728
Recurring | Consolidated Trusts
Liabilities [Abstract]
Long-term debt at fair value	22,972	24,586
Recurring | Fannie Mae
Assets [Abstract]
Trading securities	1,836	2,404
Available-for-sale securities	563	1,168
Recurring | Other agency
Assets [Abstract]
Trading securities	3,333	3,451
Available-for-sale securities	15	65
Recurring | Private-label and other mortgage securities
Assets [Abstract]
Trading securities	152	158
Available-for-sale securities	5	6
Recurring | Mortgage revenue bonds
Assets [Abstract]
Available-for-sale securities	172	216
Recurring | Other
Assets [Abstract]
Available-for-sale securities	200	242
Recurring | U.S. Treasury securities
Assets [Abstract]
Trading securities	89,707	130,456
Recurring | Other securities
Assets [Abstract]
Trading securities	73	73
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets [Abstract]
Trading securities	89,707	130,456
Available-for-sale securities	0	0
Mortgage loans held for investment, net of allowance for loan losses	0	0
Other assets [Abstract]
Derivative assets at fair value	0	0
Total nonrecurring assets at fair value	92,831	131,576
Liabilities [Abstract]
Long-term debt at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Total liabilities at fair value	0	0
Cash and Cash Equivalents, Fair Value Disclosure	3,124	1,120
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Swaps [Member]
Other assets [Abstract]
Derivative assets at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Swaptions [Member]
Other assets [Abstract]
Derivative assets at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mortgage commitment derivatives
Other assets [Abstract]
Derivative assets at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Other assets [Abstract]
Derivative assets at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fannie Mae
Liabilities [Abstract]
Long-term debt at fair value	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fannie Mae | Total senior floating
Liabilities [Abstract]
Long-term debt at fair value	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Consolidated Trusts
Liabilities [Abstract]
Long-term debt at fair value	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Fannie Mae
Assets [Abstract]
Trading securities	0	0
Available-for-sale securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other agency
Assets [Abstract]
Trading securities	0	0
Available-for-sale securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Private-label and other mortgage securities
Assets [Abstract]
Trading securities	0	0
Available-for-sale securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mortgage revenue bonds
Assets [Abstract]
Available-for-sale securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Assets [Abstract]
Available-for-sale securities	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury securities
Assets [Abstract]
Trading securities	89,707	130,456
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other securities
Assets [Abstract]
Trading securities	0	0
Recurring | Significant Other Observable Inputs (Level 2)
Assets [Abstract]
Trading securities	5,253	5,991
Available-for-sale securities	503	1,049
Mortgage loans held for investment, net of allowance for loan losses	4,784	5,629
Other assets [Abstract]
Derivative assets at fair value	500	1,748
Total nonrecurring assets at fair value	11,040	14,417
Liabilities [Abstract]
Long-term debt at fair value	25,466	27,815
Other liabilities [Abstract]
Derivative liabilities at fair value	1,335	2,441
Total liabilities at fair value	26,801	30,256
Recurring | Significant Other Observable Inputs (Level 2) | Swaps [Member]
Other assets [Abstract]
Derivative assets at fair value	118	376
Other liabilities [Abstract]
Derivative liabilities at fair value	813	881
Recurring | Significant Other Observable Inputs (Level 2) | Swaptions [Member]
Other assets [Abstract]
Derivative assets at fair value	71	383
Other liabilities [Abstract]
Derivative liabilities at fair value	46	134
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage commitment derivatives
Other assets [Abstract]
Derivative assets at fair value	311	989
Other liabilities [Abstract]
Derivative liabilities at fair value	476	1,426
Recurring | Significant Other Observable Inputs (Level 2) | Other
Other assets [Abstract]
Derivative assets at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Recurring | Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities [Abstract]
Long-term debt at fair value	2,549	3,312
Recurring | Significant Other Observable Inputs (Level 2) | Fannie Mae | Total senior floating
Liabilities [Abstract]
Long-term debt at fair value	2,549	3,312
Recurring | Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities [Abstract]
Long-term debt at fair value	22,917	24,503
Recurring | Significant Other Observable Inputs (Level 2) | Fannie Mae
Assets [Abstract]
Trading securities	1,695	2,310
Available-for-sale securities	479	973
Recurring | Significant Other Observable Inputs (Level 2) | Other agency
Assets [Abstract]
Trading securities	3,333	3,450
Available-for-sale securities	15	65
Recurring | Significant Other Observable Inputs (Level 2) | Private-label and other mortgage securities
Assets [Abstract]
Trading securities	152	158
Available-for-sale securities	3	4
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage revenue bonds
Assets [Abstract]
Available-for-sale securities	0	0
Recurring | Significant Other Observable Inputs (Level 2) | Other
Assets [Abstract]
Available-for-sale securities	6	7
Recurring | Significant Other Observable Inputs (Level 2) | U.S. Treasury securities
Assets [Abstract]
Trading securities	0	0
Recurring | Significant Other Observable Inputs (Level 2) | Other securities
Assets [Abstract]
Trading securities	73	73
Recurring | Significant Unobservable Inputs (Level 3)
Assets [Abstract]
Trading securities	141	195
Available-for-sale securities	452	648
Mortgage loans held for investment, net of allowance for loan losses	832	861
Other assets [Abstract]
Derivative assets at fair value	226	382
Total nonrecurring assets at fair value	1,651	1,986
Liabilities [Abstract]
Long-term debt at fair value	452	499
Other liabilities [Abstract]
Derivative liabilities at fair value	32	49
Total liabilities at fair value	484	548
Recurring | Significant Unobservable Inputs (Level 3) | Swaps [Member]
Other assets [Abstract]
Derivative assets at fair value	176	203
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Swaptions [Member]
Other assets [Abstract]
Derivative assets at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Mortgage commitment derivatives
Other assets [Abstract]
Derivative assets at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Other
Other assets [Abstract]
Derivative assets at fair value	50	179
Other liabilities [Abstract]
Derivative liabilities at fair value	32	49
Recurring | Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities [Abstract]
Long-term debt at fair value	397	416
Recurring | Significant Unobservable Inputs (Level 3) | Fannie Mae | Total senior floating
Liabilities [Abstract]
Long-term debt at fair value	397	416
Recurring | Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities [Abstract]
Long-term debt at fair value	55	83
Recurring | Significant Unobservable Inputs (Level 3) | Fannie Mae
Assets [Abstract]
Trading securities	141	94
Available-for-sale securities	84	195
Recurring | Significant Unobservable Inputs (Level 3) | Other agency
Assets [Abstract]
Trading securities	0	1
Available-for-sale securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Private-label and other mortgage securities
Assets [Abstract]
Trading securities	0	0
Available-for-sale securities	2	2
Recurring | Significant Unobservable Inputs (Level 3) | Mortgage revenue bonds
Assets [Abstract]
Available-for-sale securities	172	216
Recurring | Significant Unobservable Inputs (Level 3) | Other
Assets [Abstract]
Available-for-sale securities	194	235
Recurring | Significant Unobservable Inputs (Level 3) | U.S. Treasury securities
Assets [Abstract]
Trading securities	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Other securities
Assets [Abstract]
Trading securities	$ 0	0
Recurring | Significant Unobservable Inputs (Level 3) | Trading Securities
Assets [Abstract]
Trading securities		$ 95