Fair Value Levels Hierarchy (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Assets [Abstract]
Mortgage loans held for investment, at fair value	$ 11,683	$ 12,057
Other assets [Abstract]
Derivative assets, gross amount offset	(4,104)	(4,514)
Other liabilities [Abstract]
Derivative liabilities, gross amount offset	(5,397)	(6,844)
Mortgage commitment derivatives [Member]
Other assets [Abstract]
Derivative assets, 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	9,162	9,582
Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	36,372	36,524
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets [Abstract]
Trading securities	30,155	32,317
Available-for-sale securities	0	0
Mortgage loans held for investment, at fair value	0	0
Other assets [Abstract]
Derivatives assets at fair value	0	0
Total assets at fair value	62,364	73,794
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) [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	6,381	7,118
Available-for-sale securities	5,682	6,210
Mortgage loans held for investment, at fair value	2,803,139	2,767,813
Other assets [Abstract]
Derivatives assets at fair value	4,547	5,019
Total assets at fair value	2,880,520	2,844,940
Other liabilities [Abstract]
Derivative liabilities at fair value	6,257	7,921
Total liabilities at fair value	3,257,515	3,243,215
Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	296,836	298,980
Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	2,916,788	2,901,316
Significant Unobservable Inputs (Level 3) [Member]
Assets [Abstract]
Trading securities	1,148	1,127
Available-for-sale securities	2,039	2,153
Mortgage loans held for investment, at fair value	312,761	316,742
Other assets [Abstract]
Derivatives assets at fair value	156	182
Total assets at fair value	321,025	323,739
Other liabilities [Abstract]
Derivative liabilities at fair value	43	138
Total liabilities at fair value	39,359	38,870
Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	785	770
Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	37,409	36,668
Recurring Fair Value Measurements [Member]
Assets [Abstract]
Trading securities	37,684	40,562
Available-for-sale securities	7,721	8,363
Mortgage loans held for investment, at fair value	11,683	12,057
Other assets [Abstract]
Derivatives assets at fair value	599	687
Derivative assets, gross amount offset	(4,104)	(4,514)
Total assets at fair value	57,687	61,669
Liabilities [Abstract]
Long-term debt, fair value	45,534	46,106
Other liabilities [Abstract]
Derivative liabilities at fair value	903	1,215
Derivative liabilities, gross amount offset	(5,397)	(6,844)
Total liabilities at fair value	46,437	47,321
Recurring Fair Value Measurements [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	4,029	4,296
Other liabilities [Abstract]
Derivative liabilities at fair value	5,446	6,981
Recurring Fair Value Measurements [Member] | Swaptions [Member]
Other assets [Abstract]
Derivatives assets at fair value	230	241
Other liabilities [Abstract]
Derivative liabilities at fair value	270	339
Recurring Fair Value Measurements [Member] | Other [Member]
Other assets [Abstract]
Derivatives assets at fair value	27	33
Other liabilities [Abstract]
Derivative liabilities at fair value	1	2
Recurring Fair Value Measurements [Member] | Mortgage commitment derivatives [Member]
Other assets [Abstract]
Derivatives assets at fair value	417	631
Other liabilities [Abstract]
Derivative liabilities at fair value	583	737
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	9,162	9,582
Recurring Fair Value Measurements [Member] | Fannie Mae [Member] | Senior floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	9,162	9,582
Recurring Fair Value Measurements [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	36,372	36,524
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	4,594	4,769
Available-for-sale securities	2,373	2,554
Recurring Fair Value Measurements [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	2,053	2,058
Available-for-sale securities	470	547
Recurring Fair Value Measurements [Member] | Alt-A and subprime private-label securities [Member]
Assets [Abstract]
Trading securities	624	636
Available-for-sale securities	2,628	2,709
Recurring Fair Value Measurements [Member] | CMBS [Member]
Assets [Abstract]
Trading securities	238	761
Available-for-sale securities	617	819
Recurring Fair Value Measurements [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Trading securities	20	21
Available-for-sale securities	1,185	1,272
Recurring Fair Value Measurements [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	448	462
Recurring Fair Value Measurements [Member] | U.S. Treasury securities [Member]
Assets [Abstract]
Trading securities	30,155	32,317
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets [Abstract]
Trading securities	30,155	32,317
Available-for-sale securities	0	0
Mortgage loans held for investment, at fair value	0	0
Other assets [Abstract]
Derivatives assets at fair value	0	0
Total assets at fair value	30,155	32,317
Liabilities [Abstract]
Long-term debt, fair value	0	0
Other liabilities [Abstract]
Derivative 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]
Derivatives assets at fair value	0	0
Other liabilities [Abstract]
Derivative 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]
Derivatives assets at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other [Member]
Other assets [Abstract]
Derivatives assets at fair value	0	0
Other liabilities [Abstract]
Derivative 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 [Member]
Other assets [Abstract]
Derivatives assets at fair value	0	0
Other liabilities [Abstract]
Derivative 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]
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]
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 [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	30,155	32,317
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets [Abstract]
Trading securities	6,381	7,118
Available-for-sale securities	5,682	6,210
Mortgage loans held for investment, at fair value	10,534	10,860
Other assets [Abstract]
Derivatives assets at fair value	4,547	5,019
Total assets at fair value	27,144	29,207
Liabilities [Abstract]
Long-term debt, fair value	44,970	45,518
Other liabilities [Abstract]
Derivative liabilities at fair value	6,257	7,921
Total liabilities at fair value	51,227	53,439
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	3,903	4,159
Other liabilities [Abstract]
Derivative liabilities at fair value	5,408	6,933
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaptions [Member]
Other assets [Abstract]
Derivatives assets at fair value	230	241
Other liabilities [Abstract]
Derivative liabilities at fair value	270	339
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Other [Member]
Other assets [Abstract]
Derivatives assets at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mortgage commitment derivatives [Member]
Other assets [Abstract]
Derivatives assets at fair value	414	619
Other liabilities [Abstract]
Derivative liabilities at fair value	579	649
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	8,812	9,235
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	8,812	9,235
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	36,158	36,283
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	3,738	3,934
Available-for-sale securities	2,141	2,324
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	2,053	2,058
Available-for-sale securities	470	542
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alt-A and subprime private-label securities [Member]
Assets [Abstract]
Trading securities	352	365
Available-for-sale securities	2,423	2,492
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | CMBS [Member]
Assets [Abstract]
Trading securities	238	761
Available-for-sale securities	617	819
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Trading securities	0	0
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	31	33
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 Unobservable Inputs (Level 3) [Member]
Assets [Abstract]
Trading securities	1,148	1,127
Available-for-sale securities	2,039	2,153
Mortgage loans held for investment, at fair value	1,149	1,197
Other assets [Abstract]
Derivatives assets at fair value	156	182
Total assets at fair value	4,492	4,659
Liabilities [Abstract]
Long-term debt, fair value	564	588
Other liabilities [Abstract]
Derivative liabilities at fair value	43	138
Total liabilities at fair value	607	726
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Swaps [Member]
Other assets [Abstract]
Derivatives assets at fair value	126	137
Other liabilities [Abstract]
Derivative liabilities at fair value	38	48
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Swaptions [Member]
Other assets [Abstract]
Derivatives assets at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other [Member]
Other assets [Abstract]
Derivatives assets at fair value	27	33
Other liabilities [Abstract]
Derivative liabilities at fair value	1	2
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mortgage commitment derivatives [Member]
Other assets [Abstract]
Derivatives assets at fair value	3	12
Other liabilities [Abstract]
Derivative liabilities at fair value	4	88
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	350	347
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member] | Senior floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	350	347
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	214	241
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	856	835
Available-for-sale securities	232	230
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other agency [Member]
Assets [Abstract]
Trading securities	0	0
Available-for-sale securities	0	5
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Alt-A and subprime private-label securities [Member]
Assets [Abstract]
Trading securities	272	271
Available-for-sale securities	205	217
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | CMBS [Member]
Assets [Abstract]
Trading securities	0	0
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Trading securities	20	21
Available-for-sale securities	1,185	1,272
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	417	429
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | U.S. Treasury securities [Member]
Assets [Abstract]
Trading securities	$ 0	$ 0