Fair Value Levels Hierarchy (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Assets [Abstract]
Mortgage loans held for investment, at fair value	$ 8,922	$ 10,596
Other assets [Abstract]
Derivative asset, gross amount offset	(2,266)	(4,272)
Other liabilities [Abstract]
Derivative liabilities, gross amount offset	(2,315)	(4,979)
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	6,826	8,186
Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	23,753	30,493
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets [Abstract]
Trading securities	35,502	29,222
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	69,575	64,282
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	6,332	4,256
Available-for-sale securities	2,477	3,530
Mortgage loans held for investment, at fair value	2,990,104	2,886,470
Other assets [Abstract]
Derivative assets at fair value	2,515	4,274
Total assets at fair value	3,053,308	2,948,237
Other liabilities [Abstract]
Derivatives liabilities at fair value	3,077	5,272
Total liabilities at fair value	3,303,620	3,302,681
Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	211,403	249,780
Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	3,064,239	3,014,250
Significant Unobservable Inputs (Level 3) [Member]
Assets [Abstract]
Trading securities	33	1,201
Available-for-sale securities	952	1,313
Mortgage loans held for investment, at fair value	216,404	315,719
Other assets [Abstract]
Derivative assets at fair value	209	169
Total assets at fair value	225,812	324,173
Other liabilities [Abstract]
Derivatives liabilities at fair value	15	35
Total liabilities at fair value	39,950	42,389
Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	771	837
Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	39,043	40,683
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	41,867	34,679
Available-for-sale securities	3,429	4,843
Mortgage loans held for investment, at fair value	8,922	10,596
Other assets [Abstract]
Derivative asset, gross amount offset	(2,266)	(4,272)
Derivative assets at fair value	458	171
Total assets at fair value	55,424	50,289
Liabilities [Abstract]
Long-term debt, fair value	30,579	38,679
Other liabilities [Abstract]
Derivatives liabilities at fair value	777	328
Derivative liabilities, gross amount offset	(2,315)	(4,979)
Total liabilities at fair value	31,356	39,007
Recurring Fair Value Measurements [Member] | Swaps [Member]
Other assets [Abstract]
Derivative assets at fair value	2,077	4,181
Other liabilities [Abstract]
Derivatives liabilities at fair value	2,091	4,754
Recurring Fair Value Measurements [Member] | Swaptions [Member]
Other assets [Abstract]
Derivative assets at fair value	211	108
Other liabilities [Abstract]
Derivatives liabilities at fair value	342	324
Recurring Fair Value Measurements [Member] | Credit enhancement derivatives
Other assets [Abstract]
Derivative assets at fair value	57	22
Other liabilities [Abstract]
Derivatives liabilities at fair value	11	1
Recurring Fair Value Measurements [Member] | Mortgage commitment derivatives
Other assets [Abstract]
Derivative assets at fair value	379	132
Other liabilities [Abstract]
Derivatives liabilities at fair value	648	228
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	6,826	8,186
Recurring Fair Value Measurements [Member] | Fannie Mae [Member] | Senior floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	6,826	8,186
Recurring Fair Value Measurements [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	23,753	30,493
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	1,467	3,876
Available-for-sale securities	1,797	2,119
Recurring Fair Value Measurements [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	3,503	1,118
Available-for-sale securities	256	357
Recurring Fair Value Measurements [Member] | Alt-A and subprime private-label securities [Member]
Assets [Abstract]
Trading securities	1,306	463
Available-for-sale securities	592	1,314
Recurring Fair Value Measurements [Member] | CMBS [Member]
Assets [Abstract]
Available-for-sale securities		15
Recurring Fair Value Measurements [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Available-for-sale securities	434	671
Recurring Fair Value Measurements [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	350	367
Recurring Fair Value Measurements [Member] | U.S. Treasury Securities [Member]
Assets [Abstract]
Trading securities	35,502	29,222
Recurring Fair Value Measurements [Member] | Debt Security, Corporate, US [Member]
Assets [Abstract]
Trading securities	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	35,502	29,222
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	36,250	29,222
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] | CMBS [Member]
Assets [Abstract]
Available-for-sale securities		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	35,502	29,222
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
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	6,332	4,256
Available-for-sale securities	2,477	3,530
Mortgage loans held for investment, at fair value	7,985	9,480
Other assets [Abstract]
Derivative assets at fair value	2,515	4,274
Total assets at fair value	19,309	21,540
Liabilities [Abstract]
Long-term debt, fair value	30,027	37,721
Other liabilities [Abstract]
Derivatives liabilities at fair value	3,077	5,272
Total liabilities at fair value	33,104	42,993
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaps [Member]
Other assets [Abstract]
Derivative assets at fair value	1,962	4,035
Other liabilities [Abstract]
Derivatives liabilities at fair value	2,089	4,721
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaptions [Member]
Other assets [Abstract]
Derivative assets at fair value	211	108
Other liabilities [Abstract]
Derivatives liabilities at fair value	342	324
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	342	131
Other liabilities [Abstract]
Derivatives liabilities at fair value	646	227
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	6,475	7,810
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	6,475	7,810
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	23,552	29,911
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	1,435	2,905
Available-for-sale securities	1,645	1,911
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	3,503	1,083
Available-for-sale securities	256	357
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alt-A and subprime private-label securities [Member]
Assets [Abstract]
Trading securities	1,305	268
Available-for-sale securities	568	1,237
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | CMBS [Member]
Assets [Abstract]
Available-for-sale securities		15
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	10
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	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	33	1,201
Available-for-sale securities	952	1,313
Mortgage loans held for investment, at fair value	937	1,116
Other assets [Abstract]
Derivative assets at fair value	209	169
Total assets at fair value	2,131	3,799
Liabilities [Abstract]
Long-term debt, fair value	552	958
Other liabilities [Abstract]
Derivatives liabilities at fair value	15	35
Total liabilities at fair value	567	993
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Swaps [Member]
Other assets [Abstract]
Derivative assets at fair value	115	146
Other liabilities [Abstract]
Derivatives liabilities at fair value	2	33
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	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Credit enhancement derivatives
Other assets [Abstract]
Derivative assets at fair value	57	22
Other liabilities [Abstract]
Derivatives liabilities at fair value	11	1
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mortgage commitment derivatives
Other assets [Abstract]
Derivative assets at fair value	37	1
Other liabilities [Abstract]
Derivatives liabilities at fair value	2	1
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	351	376
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member] | Senior floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	351	376
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	201	582
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	32	971
Available-for-sale securities	152	208
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	0	35
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	1	195
Available-for-sale securities	24	77
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | CMBS [Member]
Assets [Abstract]
Available-for-sale securities		0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Available-for-sale securities	434	671
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	342	357
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