Fair Value Levels Hierarchy (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Assets [Abstract]
Mortgage loans held for investment, at fair value	$ 7,825	$ 8,922
Other assets [Abstract]
Derivative asset, gross amount offset	(1,288)	(2,266)
Other liabilities [Abstract]
Derivative liabilities, gross amount offset	(1,694)	(2,315)
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	5,687	6,826
Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	21,880	23,753
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets [Abstract]
Trading securities	39,501	35,502
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 assets at fair value	89,558	69,575
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	8,576	6,332
Available-for-sale securities	1,612	2,477
Mortgage loans held for investment, at fair value	3,270,535	2,990,104
Other assets [Abstract]
Derivative assets at fair value	1,360	2,515
Total assets at fair value	3,313,691	3,053,308
Other liabilities [Abstract]
Derivatives liabilities at fair value	2,092	3,077
Total liabilities at fair value	3,506,144	3,303,620
Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	164,144	211,403
Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	3,312,763	3,064,239
Significant Unobservable Inputs (Level 3) [Member]
Assets [Abstract]
Trading securities	46	33
Available-for-sale securities	792	952
Mortgage loans held for investment, at fair value	127,650	216,404
Other assets [Abstract]
Derivative assets at fair value	199	209
Total assets at fair value	136,048	225,812
Other liabilities [Abstract]
Derivatives liabilities at fair value	37	15
Total liabilities at fair value	32,362	39,950
Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	401	771
Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	31,827	39,043
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		748
Assets [Abstract]
Trading securities	48,123	41,867
Available-for-sale securities	2,404	3,429
Mortgage loans held for investment, at fair value	7,825	8,922
Other assets [Abstract]
Derivative asset, gross amount offset	(1,288)	(2,266)
Derivative assets at fair value	271	458
Total assets at fair value	58,623	55,424
Liabilities [Abstract]
Long-term debt, fair value	27,567	30,579
Other liabilities [Abstract]
Derivatives liabilities at fair value	435	777
Derivative liabilities, gross amount offset	(1,694)	(2,315)
Total liabilities at fair value	28,002	31,356
Recurring Fair Value Measurements [Member] | Swaps [Member]
Other assets [Abstract]
Derivative assets at fair value	1,230	2,077
Other liabilities [Abstract]
Derivatives liabilities at fair value	1,347	2,091
Recurring Fair Value Measurements [Member] | Swaptions [Member]
Other assets [Abstract]
Derivative assets at fair value	124	211
Other liabilities [Abstract]
Derivatives liabilities at fair value	451	342
Recurring Fair Value Measurements [Member] | Credit enhancement derivatives
Other assets [Abstract]
Derivative assets at fair value	40	57
Other liabilities [Abstract]
Derivatives liabilities at fair value	25	11
Recurring Fair Value Measurements [Member] | Mortgage commitment derivatives
Other assets [Abstract]
Derivative assets at fair value	165	379
Other liabilities [Abstract]
Derivatives liabilities at fair value	306	648
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	5,687	6,826
Recurring Fair Value Measurements [Member] | Fannie Mae [Member] | Senior floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	5,687	6,826
Recurring Fair Value Measurements [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	21,880	23,753
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	3,424	1,467
Available-for-sale securities	1,520	1,797
Recurring Fair Value Measurements [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	4,490	3,503
Available-for-sale securities	198	256
Recurring Fair Value Measurements [Member] | Alt-A and subprime private-label securities [Member]
Assets [Abstract]
Trading securities	629	1,306
Available-for-sale securities	57	592
Recurring Fair Value Measurements [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Available-for-sale securities	315	434
Recurring Fair Value Measurements [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	314	350
Recurring Fair Value Measurements [Member] | U.S. Treasury Securities [Member]
Assets [Abstract]
Trading securities	39,501	35,502
Recurring Fair Value Measurements [Member] | Debt Security, Corporate, US [Member]
Assets [Abstract]
Trading securities	79	89
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		748
Assets [Abstract]
Trading securities	39,501	35,502
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 assets at fair value	39,501	36,250
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	39,501	35,502
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]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		0
Assets [Abstract]
Trading securities	8,576	6,332
Available-for-sale securities	1,612	2,477
Mortgage loans held for investment, at fair value	7,137	7,985
Other assets [Abstract]
Derivative assets at fair value	1,360	2,515
Total assets at fair value	18,685	19,309
Liabilities [Abstract]
Long-term debt, fair value	27,094	30,027
Other liabilities [Abstract]
Derivatives liabilities at fair value	2,092	3,077
Total liabilities at fair value	29,186	33,104
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaps [Member]
Other assets [Abstract]
Derivative assets at fair value	1,071	1,962
Other liabilities [Abstract]
Derivatives liabilities at fair value	1,346	2,089
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaptions [Member]
Other assets [Abstract]
Derivative assets at fair value	124	211
Other liabilities [Abstract]
Derivatives liabilities at fair value	440	342
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	165	342
Other liabilities [Abstract]
Derivatives liabilities at fair value	306	646
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	5,289	6,475
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	5,289	6,475
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	21,805	23,552
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	3,379	1,435
Available-for-sale securities	1,349	1,645
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	4,489	3,503
Available-for-sale securities	198	256
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alt-A and subprime private-label securities [Member]
Assets [Abstract]
Trading securities	629	1,305
Available-for-sale securities	57	568
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	8	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	79	89
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents, Fair Value Disclosure		0
Assets [Abstract]
Trading securities	46	33
Available-for-sale securities	792	952
Mortgage loans held for investment, at fair value	688	937
Other assets [Abstract]
Derivative assets at fair value	199	209
Total assets at fair value	1,725	2,131
Liabilities [Abstract]
Long-term debt, fair value	473	552
Other liabilities [Abstract]
Derivatives liabilities at fair value	37	15
Total liabilities at fair value	510	567
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Swaps [Member]
Other assets [Abstract]
Derivative assets at fair value	159	115
Other liabilities [Abstract]
Derivatives liabilities at fair value	1	2
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	11	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Credit enhancement derivatives
Other assets [Abstract]
Derivative assets at fair value	40	57
Other liabilities [Abstract]
Derivatives liabilities at fair value	25	11
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mortgage commitment derivatives
Other assets [Abstract]
Derivative assets at fair value	0	37
Other liabilities [Abstract]
Derivatives liabilities at fair value	0	2
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	398	351
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member] | Senior floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	398	351
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	75	201
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	45	32
Available-for-sale securities	171	152
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	1	0
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	1
Available-for-sale securities	0	24
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Available-for-sale securities	315	434
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	306	342
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