Fair Value Levels Hierarchy (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Assets [Abstract]
Mortgage loans held for investment, at fair value	$ 13,413	$ 14,075
Other assets [Abstract]
Derivative assets, gross amount offset	(7,530)	(4,024)
Other liabilities [Abstract]
Derivative liabilities, gross amount offset	(14,269)	(8,650)
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	10,650	11,133
Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	32,798	23,609
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets [Abstract]
Trading securities	29,652	29,485
Available-for-sale securities	0	0
Mortgage loans held for investment, at fair value	0	0
Total assets at fair value	71,848	64,438
Other assets [Abstract]
Derivatives assets at fair value	0	0
Liabilities [Abstract]
Total liabilities at fair value	0	0
Other liabilities [Abstract]
Derivative liabilities at fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fannie Mae [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	0	0
Liabilities [Abstract]
Long-term debt, fair value	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Consolidated Trusts [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	0	0
Liabilities [Abstract]
Long-term debt, fair value	0	0
Significant Other Observable Inputs (Level 2) [Member]
Assets [Abstract]
Trading securities	9,019	9,025
Available-for-sale securities	10,352	11,799
Mortgage loans held for investment, at fair value	2,705,804	2,702,526
Total assets at fair value	2,779,984	2,770,088
Other assets [Abstract]
Derivatives assets at fair value	8,048	4,729
Liabilities [Abstract]
Total liabilities at fair value	3,292,266	3,224,091
Other liabilities [Abstract]
Derivative liabilities at fair value	15,395	9,042
Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	29,508	26,544
Liabilities [Abstract]
Long-term debt, fair value	315,023	324,248
Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	2,676,296	2,675,982
Liabilities [Abstract]
Long-term debt, fair value	2,901,213	2,819,733
Significant Unobservable Inputs (Level 3) [Member]
Assets [Abstract]
Trading securities	496	1,398
Available-for-sale securities	2,828	8,431
Mortgage loans held for investment, at fair value	415,125	351,355
Total assets at fair value	423,793	367,852
Other assets [Abstract]
Derivatives assets at fair value	257	189
Liabilities [Abstract]
Total liabilities at fair value	42,660	30,061
Other liabilities [Abstract]
Derivative liabilities at fair value	9	32
Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	183,942	193,670
Liabilities [Abstract]
Long-term debt, fair value	881	898
Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Assets [Abstract]
Mortgage loans held for investment, at fair value	231,183	157,685
Liabilities [Abstract]
Long-term debt, fair value	40,138	27,175
Recurring Fair Value Measurements [Member]
Assets [Abstract]
Trading securities	39,167	39,908
Available-for-sale securities	13,180	20,230
Mortgage loans held for investment, at fair value	13,413	14,075
Total assets at fair value	66,535	75,107
Other assets [Abstract]
Derivatives assets at fair value	775	894
Derivative assets, gross amount offset	(7,530)	(4,024)
Liabilities [Abstract]
Long-term debt, fair value	43,448	34,742
Total liabilities at fair value	44,583	35,166
Other liabilities [Abstract]
Derivative liabilities at fair value	1,135	424
Derivative liabilities, gross amount offset	(14,269)	(8,650)
Recurring Fair Value Measurements [Member] | Interest Rate Swaps
Other assets [Abstract]
Derivatives assets at fair value	7,605	4,697
Other liabilities [Abstract]
Derivative liabilities at fair value	14,287	8,717
Recurring Fair Value Measurements [Member] | Swaptions [Member]
Other assets [Abstract]
Derivatives assets at fair value	36	53
Other liabilities [Abstract]
Derivative liabilities at fair value	158	197
Recurring Fair Value Measurements [Member] | Other [Member]
Other assets [Abstract]
Derivatives assets at fair value	27	28
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	637	140
Other liabilities [Abstract]
Derivative liabilities at fair value	958	158
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	10,650	11,133
Recurring Fair Value Measurements [Member] | Fannie Mae [Member] | Senior floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	10,650	11,133
Recurring Fair Value Measurements [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	32,798	23,609
Recurring Fair Value Measurements [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	5,352	4,813
Available-for-sale securities	3,336	4,221
Recurring Fair Value Measurements [Member] | Freddie Mac [Member]
Assets [Abstract]
Trading securities	1,328	1,314
Available-for-sale securities	2,038	4,299
Recurring Fair Value Measurements [Member] | Ginnie Mae [Member]
Assets [Abstract]
Trading securities	542	426
Available-for-sale securities	66	391
Recurring Fair Value Measurements [Member] | Alt-A private-label securities [Member]
Assets [Abstract]
Trading securities	316	436
Available-for-sale securities	1,827	2,678
Recurring Fair Value Measurements [Member] | Subprime private-label securities [Member]
Assets [Abstract]
Trading securities	395	644
Available-for-sale securities	2,373	3,281
Recurring Fair Value Measurements [Member] | CMBS [Member]
Assets [Abstract]
Trading securities	1,389	2,341
Available-for-sale securities	1,023	1,255
Recurring Fair Value Measurements [Member] | Mortgage revenue bonds [Member]
Assets [Abstract]
Trading securities	193	449
Available-for-sale securities	2,029	2,701
Recurring Fair Value Measurements [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	488	1,404
Recurring Fair Value Measurements [Member] | U.S. Treasury securities [Member]
Assets [Abstract]
Trading securities	29,652	29,485
Recurring Fair Value Measurements [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Assets [Abstract]
Trading securities	29,652	29,485
Available-for-sale securities	0	0
Mortgage loans held for investment, at fair value	0	0
Total assets at fair value	29,652	29,485
Other assets [Abstract]
Derivatives assets at fair value	0	0
Liabilities [Abstract]
Long-term debt, fair value	0	0
Total liabilities 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] | Interest Rate Swaps
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] | Freddie Mac [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] | Ginnie 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] | Alt-A 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] | 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	29,652	29,485
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member]
Assets [Abstract]
Trading securities	9,019	9,025
Available-for-sale securities	10,352	11,799
Mortgage loans held for investment, at fair value	12,133	12,598
Total assets at fair value	39,552	38,151
Other assets [Abstract]
Derivatives assets at fair value	8,048	4,729
Liabilities [Abstract]
Long-term debt, fair value	42,712	33,877
Total liabilities at fair value	58,107	42,919
Other liabilities [Abstract]
Derivative liabilities at fair value	15,395	9,042
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Interest Rate Swaps
Other assets [Abstract]
Derivatives assets at fair value	7,389	4,541
Other liabilities [Abstract]
Derivative liabilities at fair value	14,283	8,697
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Swaptions [Member]
Other assets [Abstract]
Derivatives assets at fair value	36	53
Other liabilities [Abstract]
Derivative liabilities at fair value	158	197
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	623	135
Other liabilities [Abstract]
Derivative liabilities at fair value	954	148
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	10,240	10,764
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	10,240	10,764
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	32,472	23,113
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	5,352	4,813
Available-for-sale securities	3,336	4,221
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Freddie Mac [Member]
Assets [Abstract]
Trading securities	1,328	1,314
Available-for-sale securities	2,037	4,295
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Ginnie Mae [Member]
Assets [Abstract]
Trading securities	542	426
Available-for-sale securities	66	391
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Alt-A private-label securities [Member]
Assets [Abstract]
Trading securities	57	131
Available-for-sale securities	1,652	1,637
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | Subprime private-label securities [Member]
Assets [Abstract]
Trading securities	351	0
Available-for-sale securities	2,200	0
Recurring Fair Value Measurements [Member] | Significant Other Observable Inputs (Level 2) [Member] | CMBS [Member]
Assets [Abstract]
Trading securities	1,389	2,341
Available-for-sale securities	1,023	1,255
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	38	0
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	496	1,398
Available-for-sale securities	2,828	8,431
Mortgage loans held for investment, at fair value	1,280	1,477
Total assets at fair value	4,861	11,495
Other assets [Abstract]
Derivatives assets at fair value	257	189
Liabilities [Abstract]
Long-term debt, fair value	736	865
Total liabilities at fair value	745	897
Other liabilities [Abstract]
Derivative liabilities at fair value	9	32
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Interest Rate Swaps
Other assets [Abstract]
Derivatives assets at fair value	216	156
Other liabilities [Abstract]
Derivative liabilities at fair value	4	20
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	28
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	14	5
Other liabilities [Abstract]
Derivative liabilities at fair value	4	10
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Liabilities [Abstract]
Long-term debt, fair value	410	369
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member] | Senior floating [Member]
Liabilities [Abstract]
Long-term debt, fair value	410	369
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Consolidated Trusts [Member]
Liabilities [Abstract]
Long-term debt, fair value	326	496
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fannie Mae [Member]
Assets [Abstract]
Trading securities	0	0
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Freddie Mac [Member]
Assets [Abstract]
Trading securities	0	0
Available-for-sale securities	1	4
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Ginnie Mae [Member]
Assets [Abstract]
Trading securities	0	0
Available-for-sale securities	0	0
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Alt-A private-label securities [Member]
Assets [Abstract]
Trading securities	259	305
Available-for-sale securities	175	1,041
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Subprime private-label securities [Member]
Assets [Abstract]
Trading securities	44	644
Available-for-sale securities	173	3,281
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	193	449
Available-for-sale securities	2,029	2,701
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | Other [Member]
Assets [Abstract]
Available-for-sale securities	450	1,404
Recurring Fair Value Measurements [Member] | Significant Unobservable Inputs (Level 3) [Member] | U.S. Treasury securities [Member]
Assets [Abstract]
Trading securities	$ 0	$ 0