Fair Value - Levels Hierarchy (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Assets [Abstract]
Available-for-sale, at fair value	$ 886	$ 1,697
Mortgage loans held for investment, net of allowance for loan losses	5,301	6,490
Other assets [Abstract]
Derivative assets, gross amount offset	(290)	(905)
Other liabilities [Abstract]
Derivative liabilities, gross amount offset	(838)	(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	2,633	3,728
Consolidated Trusts
Liabilities [Abstract]
Long-term debt at fair value	23,020	24,586
Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets [Abstract]
Trading securities	92,192	130,456
Available-for-sale, at fair value	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	166,256	227,635
Other liabilities [Abstract]
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 [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	4,888	5,991
Available-for-sale, at fair value	90	1,049
Mortgage loans held for investment, net of allowance for loan losses	3,745,286	3,512,672
Other assets [Abstract]
Derivative assets at fair value	940	1,748
Total nonrecurring assets at fair value	3,850,387	3,578,668
Other liabilities [Abstract]
Derivative liabilities at fair value	1,440	2,441
Total financial liabilities	4,182,586	4,059,705
Significant Other Observable Inputs (Level 2) | Fannie Mae
Liabilities [Abstract]
Long-term debt at fair value	235,120	288,414
Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities [Abstract]
Long-term debt at fair value	3,937,618	3,756,673
Significant Unobservable Inputs (Level 3)
Assets [Abstract]
Trading securities	129	95
Available-for-sale, at fair value	796	648
Mortgage loans held for investment, net of allowance for loan losses	220,358	255,556
Other assets [Abstract]
Derivative assets at fair value	159	382
Total nonrecurring assets at fair value	227,806	262,442
Other liabilities [Abstract]
Derivative liabilities at fair value	20	49
Total financial liabilities	31,834	32,593
Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities [Abstract]
Long-term debt at fair value	814	878
Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities [Abstract]
Long-term debt at fair value	30,914	31,584
Recurring
Assets [Abstract]
Cash equivalents	875	1,120
Trading securities	97,209	136,542
Available-for-sale, at fair value	886	1,697
Mortgage loans held for investment, net of allowance for loan losses	5,301	6,490
Other assets [Abstract]
Derivative assets at fair value	809	1,225
Derivative assets, gross amount offset	(290)	(905)
Total nonrecurring assets at fair value	105,080	147,074
Liabilities [Abstract]
Long-term debt at fair value	25,653	28,314
Other liabilities [Abstract]
Derivative liabilities at fair value	622	1,495
Derivative liabilities, gross amount offset	(838)	(995)
Total financial liabilities	26,275	29,809
Recurring | Swaps
Other assets [Abstract]
Derivative assets at fair value	261	579
Other liabilities [Abstract]
Derivative liabilities at fair value	816	881
Recurring | Swaptions
Other assets [Abstract]
Derivative assets at fair value	75	383
Other liabilities [Abstract]
Derivative liabilities at fair value	25	134
Recurring | Mortgage commitment derivatives
Other assets [Abstract]
Derivative assets at fair value	763	989
Other liabilities [Abstract]
Derivative liabilities at fair value	599	1,426
Recurring | Other
Other assets [Abstract]
Derivative assets at fair value	0	179
Other liabilities [Abstract]
Derivative liabilities at fair value	20	49
Recurring | Fannie Mae
Liabilities [Abstract]
Long-term debt at fair value	2,633	3,728
Recurring | Fannie Mae | Total senior floating
Liabilities [Abstract]
Long-term debt at fair value	2,633	3,728
Recurring | Consolidated Trusts
Liabilities [Abstract]
Long-term debt at fair value	23,020	24,586
Recurring | Fannie Mae
Assets [Abstract]
Trading securities	1,715	2,404
Available-for-sale, at fair value	518	1,168
Recurring | Other agency
Assets [Abstract]
Trading securities	3,139	3,451
Available-for-sale, at fair value	15	65
Recurring | Private-label and other mortgage securities
Assets [Abstract]
Trading securities	143	158
Available-for-sale, at fair value	3	6
Recurring | Mortgage revenue bonds
Assets [Abstract]
Available-for-sale, at fair value	164	216
Recurring | Other
Assets [Abstract]
Available-for-sale, at fair value	186	242
Recurring | U.S. Treasury securities
Assets [Abstract]
Trading securities	92,192	130,456
Recurring | Other securities
Assets [Abstract]
Trading securities	20	73
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Assets [Abstract]
Cash equivalents	875	1,120
Trading securities	92,192	130,456
Available-for-sale, at fair value	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	93,067	131,576
Liabilities [Abstract]
Long-term debt at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Total financial liabilities	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Swaps
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
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, at fair value	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other agency
Assets [Abstract]
Trading securities	0	0
Available-for-sale, at fair value	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, at fair value	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Mortgage revenue bonds
Assets [Abstract]
Available-for-sale, at fair value	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Other
Assets [Abstract]
Available-for-sale, at fair value	0	0
Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury securities
Assets [Abstract]
Trading securities	92,192	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	4,888	5,991
Available-for-sale, at fair value	90	1,049
Mortgage loans held for investment, net of allowance for loan losses	4,516	5,629
Other assets [Abstract]
Derivative assets at fair value	940	1,748
Total nonrecurring assets at fair value	10,434	14,417
Liabilities [Abstract]
Long-term debt at fair value	25,164	27,815
Other liabilities [Abstract]
Derivative liabilities at fair value	1,440	2,441
Total financial liabilities	26,604	30,256
Recurring | Significant Other Observable Inputs (Level 2) | Swaps
Other assets [Abstract]
Derivative assets at fair value	102	376
Other liabilities [Abstract]
Derivative liabilities at fair value	816	881
Recurring | Significant Other Observable Inputs (Level 2) | Swaptions
Other assets [Abstract]
Derivative assets at fair value	75	383
Other liabilities [Abstract]
Derivative liabilities at fair value	25	134
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage commitment derivatives
Other assets [Abstract]
Derivative assets at fair value	763	989
Other liabilities [Abstract]
Derivative liabilities at fair value	599	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	2,245	3,312
Recurring | Significant Other Observable Inputs (Level 2) | Fannie Mae | Total senior floating
Liabilities [Abstract]
Long-term debt at fair value	2,245	3,312
Recurring | Significant Other Observable Inputs (Level 2) | Consolidated Trusts
Liabilities [Abstract]
Long-term debt at fair value	22,919	24,503
Recurring | Significant Other Observable Inputs (Level 2) | Fannie Mae
Assets [Abstract]
Trading securities	1,586	2,310
Available-for-sale, at fair value	68	973
Recurring | Significant Other Observable Inputs (Level 2) | Other agency
Assets [Abstract]
Trading securities	3,139	3,450
Available-for-sale, at fair value	15	65
Recurring | Significant Other Observable Inputs (Level 2) | Private-label and other mortgage securities
Assets [Abstract]
Trading securities	143	158
Available-for-sale, at fair value	3	4
Recurring | Significant Other Observable Inputs (Level 2) | Mortgage revenue bonds
Assets [Abstract]
Available-for-sale, at fair value	0	0
Recurring | Significant Other Observable Inputs (Level 2) | Other
Assets [Abstract]
Available-for-sale, at fair value	4	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	20	73
Recurring | Significant Unobservable Inputs (Level 3)
Assets [Abstract]
Trading securities	129	95
Available-for-sale, at fair value	796	648
Mortgage loans held for investment, net of allowance for loan losses	785	861
Other assets [Abstract]
Derivative assets at fair value	159	382
Total nonrecurring assets at fair value	1,869	1,986
Liabilities [Abstract]
Long-term debt at fair value	489	499
Other liabilities [Abstract]
Derivative liabilities at fair value	20	49
Total financial liabilities	509	548
Recurring | Significant Unobservable Inputs (Level 3) | Swaps
Other assets [Abstract]
Derivative assets at fair value	159	203
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Swaptions
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	0	179
Other liabilities [Abstract]
Derivative liabilities at fair value	20	49
Recurring | Significant Unobservable Inputs (Level 3) | Fannie Mae
Liabilities [Abstract]
Long-term debt at fair value	388	416
Recurring | Significant Unobservable Inputs (Level 3) | Fannie Mae | Total senior floating
Liabilities [Abstract]
Long-term debt at fair value	388	416
Recurring | Significant Unobservable Inputs (Level 3) | Consolidated Trusts
Liabilities [Abstract]
Long-term debt at fair value	101	83
Recurring | Significant Unobservable Inputs (Level 3) | Fannie Mae
Assets [Abstract]
Trading securities	129	94
Available-for-sale, at fair value	450	195
Recurring | Significant Unobservable Inputs (Level 3) | Other agency
Assets [Abstract]
Trading securities	0	1
Available-for-sale, at fair value	0	0
Recurring | Significant Unobservable Inputs (Level 3) | Private-label and other mortgage securities
Assets [Abstract]
Trading securities	0	0
Available-for-sale, at fair value	0	2
Recurring | Significant Unobservable Inputs (Level 3) | Mortgage revenue bonds
Assets [Abstract]
Available-for-sale, at fair value	164	216
Recurring | Significant Unobservable Inputs (Level 3) | Other
Assets [Abstract]
Available-for-sale, at fair value	182	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