Fair Value Levels Hierarchy (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets [Abstract]
Mortgage loans held for investment, at fair value	$ 6,490	$ 7,825
Other assets [Abstract]
Derivative asset, gross amount offset	(905)	(1,288)
Other liabilities [Abstract]
Derivative liabilities, gross amount offset	(995)	(1,694)
Mortgage commitment derivatives
Other assets [Abstract]
Derivative asset, gross amount offset	0	0
Other liabilities [Abstract]
Derivative liabilities, gross amount offset	0	0
Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	3,728	5,687
Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	24,586	21,880
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets [Abstract]
Trading securities	130,456	39,501
Available-for-sale securities	0	0
Mortgage loans held for investment, at fair value	0	0
Other assets [Abstract]
Derivative assets at fair value	0	0
Total nonrecurring assets at fair value	227,635	89,558
Other liabilities [Abstract]
Derivatives liabilities at fair value	0	0
Total liabilities at fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	0	0
Significant Other Observable Inputs (Level 2) [Member]
Assets [Abstract]
Trading securities	5,991	8,576
Available-for-sale securities	1,049	1,612
Mortgage loans held for investment, at fair value	3,512,672	3,270,535
Other assets [Abstract]
Derivative assets at fair value	1,748	1,360
Total nonrecurring assets at fair value	3,578,668	3,313,691
Other liabilities [Abstract]
Derivatives liabilities at fair value	2,441	2,092
Total liabilities at fair value	4,059,705	3,506,144
Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	288,414	164,144
Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	3,756,673	3,312,763
Significant Unobservable Inputs (Level 3) [Member]
Assets [Abstract]
Trading securities	95	46
Available-for-sale securities	648	792
Mortgage loans held for investment, at fair value	255,556	127,650
Other assets [Abstract]
Derivative assets at fair value	382	199
Total nonrecurring assets at fair value	262,442	136,048
Other liabilities [Abstract]
Derivatives liabilities at fair value	49	37
Total liabilities at fair value	32,593	32,362
Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	878	401
Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	31,584	31,827
Recurring Fair Value Measurements [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	1,120
Assets [Abstract]
Trading securities	136,542	48,123
Available-for-sale securities	1,697	2,404
Mortgage loans held for investment, at fair value	6,490	7,825
Other assets [Abstract]
Derivative asset, gross amount offset	(905)	(1,288)
Derivative assets at fair value	1,225	271
Total nonrecurring assets at fair value	147,074	58,623
Liabilities [Abstract]
Long-term debt, fair value	28,314	27,567
Other liabilities [Abstract]
Derivatives liabilities at fair value	1,495	435
Derivative liabilities, gross amount offset	(995)	(1,694)
Total liabilities at fair value	29,809	28,002
Recurring Fair Value Measurements [Member] | Swaps [Member]
Other assets [Abstract]
Derivative assets at fair value	579	1,230
Other liabilities [Abstract]
Derivatives liabilities at fair value	881	1,347
Recurring Fair Value Measurements [Member] | Swaptions [Member]
Other assets [Abstract]
Derivative assets at fair value	383	124
Other liabilities [Abstract]
Derivatives liabilities at fair value	134	451
Recurring Fair Value Measurements [Member] | Credit enhancement derivatives
Other assets [Abstract]
Derivative assets at fair value	179	40
Other liabilities [Abstract]
Derivatives liabilities at fair value	49	25
Recurring Fair Value Measurements [Member] | Mortgage commitment derivatives
Other assets [Abstract]
Derivative assets at fair value	989	165
Other liabilities [Abstract]
Derivatives liabilities at fair value	1,426	306
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	3,728	5,687
Recurring Fair Value Measurements [Member] | Fannie Mae [Member] | Senior floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	3,728	5,687
Recurring Fair Value Measurements [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	24,586	21,880
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	2,404	3,424
Available-for-sale securities	1,168	1,520
Recurring Fair Value Measurements [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	3,451	4,490
Available-for-sale securities	65	198
Recurring Fair Value Measurements [Member] | Alt-A and subprime private-label securities [Member]
Assets [Abstract]
Trading securities	158	629
Available-for-sale securities	6	57
Recurring Fair Value Measurements [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Available-for-sale securities	216	315
Recurring Fair Value Measurements [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	242	314
Recurring Fair Value Measurements [Member] | U.S. Treasury Securities [Member]
Assets [Abstract]
Trading securities	130,456	39,501
Recurring Fair Value Measurements [Member] | Debt Security, Corporate, US [Member]
Assets [Abstract]
Trading securities	73	79
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure	1,120
Assets [Abstract]
Trading securities	130,456	39,501
Available-for-sale securities	0	0
Mortgage loans held for investment, at fair value	0	0
Other assets [Abstract]
Derivative assets at fair value	0	0
Total nonrecurring assets at fair value	131,576	39,501
Liabilities [Abstract]
Long-term debt, fair value	0	0
Other liabilities [Abstract]
Derivatives liabilities at fair value	0	0
Total liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Swaps [Member]
Other assets [Abstract]
Derivative assets at fair value	0	0
Other liabilities [Abstract]
Derivatives liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Swaptions [Member]
Other assets [Abstract]
Derivative assets at fair value	0	0
Other liabilities [Abstract]
Derivatives liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Credit enhancement derivatives
Other assets [Abstract]
Derivative assets at fair value	0	0
Other liabilities [Abstract]
Derivatives liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Mortgage commitment derivatives
Other assets [Abstract]
Derivative assets at fair value	0	0
Other liabilities [Abstract]
Derivatives liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fannie Mae [Member] | Senior floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	0	0
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	0	0
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Alt-A and subprime private-label securities [Member]
Assets [Abstract]
Trading securities	0	0
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | U.S. Treasury Securities [Member]
Assets [Abstract]
Trading securities	130,456	39,501
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Debt Security, Corporate, US [Member]
Assets [Abstract]
Trading securities	0	0
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets [Abstract]
Trading securities	5,991	8,576
Available-for-sale securities	1,049	1,612
Mortgage loans held for investment, at fair value	5,629	7,137
Other assets [Abstract]
Derivative assets at fair value	1,748	1,360
Total nonrecurring assets at fair value	14,417	18,685
Liabilities [Abstract]
Long-term debt, fair value	27,815	27,094
Other liabilities [Abstract]
Derivatives liabilities at fair value	2,441	2,092
Total liabilities at fair value	30,256	29,186
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaps [Member]
Other assets [Abstract]
Derivative assets at fair value	376	1,071
Other liabilities [Abstract]
Derivatives liabilities at fair value	881	1,346
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaptions [Member]
Other assets [Abstract]
Derivative assets at fair value	383	124
Other liabilities [Abstract]
Derivatives liabilities at fair value	134	440
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Credit enhancement derivatives
Other assets [Abstract]
Derivative assets at fair value	0	0
Other liabilities [Abstract]
Derivatives liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mortgage commitment derivatives
Other assets [Abstract]
Derivative assets at fair value	989	165
Other liabilities [Abstract]
Derivatives liabilities at fair value	1,426	306
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	3,312	5,289
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member] | Senior floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	3,312	5,289
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	24,503	21,805
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	2,310	3,379
Available-for-sale securities	973	1,349
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	3,450	4,489
Available-for-sale securities	65	198
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alt-A and subprime private-label securities [Member]
Assets [Abstract]
Trading securities	158	629
Available-for-sale securities	4	57
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	7	8
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | U.S. Treasury Securities [Member]
Assets [Abstract]
Trading securities	0	0
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Debt Security, Corporate, US [Member]
Assets [Abstract]
Trading securities	73	79
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member]
Assets [Abstract]
Trading securities	95	46
Available-for-sale securities	648	792
Mortgage loans held for investment, at fair value	861	688
Other assets [Abstract]
Derivative assets at fair value	382	199
Total nonrecurring assets at fair value	1,986	1,725
Liabilities [Abstract]
Long-term debt, fair value	499	473
Other liabilities [Abstract]
Derivatives liabilities at fair value	49	37
Total liabilities at fair value	548	510
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Swaps [Member]
Other assets [Abstract]
Derivative assets at fair value	203	159
Other liabilities [Abstract]
Derivatives liabilities at fair value	0	1
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Swaptions [Member]
Other assets [Abstract]
Derivative assets at fair value	0	0
Other liabilities [Abstract]
Derivatives liabilities at fair value	0	11
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Credit enhancement derivatives
Other assets [Abstract]
Derivative assets at fair value	179	40
Other liabilities [Abstract]
Derivatives liabilities at fair value	49	25
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mortgage commitment derivatives
Other assets [Abstract]
Derivative assets at fair value	0	0
Other liabilities [Abstract]
Derivatives liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	416	398
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member] | Senior floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	416	398
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	83	75
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	94	45
Available-for-sale securities	195	171
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	1	1
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Alt-A and subprime private-label securities [Member]
Assets [Abstract]
Trading securities	0	0
Available-for-sale securities	2	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Available-for-sale securities	216	315
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	235	306
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | U.S. Treasury Securities [Member]
Assets [Abstract]
Trading securities	0	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Debt Security, Corporate, US [Member]
Assets [Abstract]
Trading securities	$ 0	$ 0