Allowance for Loan Losses Rollforward by Segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Allowance for Loan Losses [Roll Forward]
Beginning balance	$ 25,819	$ 31,820	$ 27,951	$ 35,541
Provision (benefit) for loan losses	(1,336)	1,029	(2,368)	1,037
Charge-offs	(821)	(2,074)	(2,105)	(7,436)
Recoveries	115	261	235	883
Transfers	0	0	0	0
Other	22	114	86	1,125
Ending balance	23,799	31,150	23,799	31,150
Fannie Mae [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	24,557	29,873	26,510	33,117
Provision (benefit) for loan losses	(1,300)	1,274	(2,429)	1,442
Charge-offs	(792)	(2,051)	(2,050)	(7,394)
Recoveries	114	257	232	871
Transfers	192	256	444	615
Other	22	115	86	1,073
Ending balance	22,793	29,724	22,793	29,724
Consolidated Trusts [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	1,262	1,947	1,441	2,424
Provision (benefit) for loan losses	(36)	(245)	61	(405)
Charge-offs	(29)	(23)	(55)	(42)
Recoveries	1	4	3	12
Transfers	(192)	(256)	(444)	(615)
Other	0	(1)	0	52
Ending balance	1,006	1,426	1,006	1,426
Single-family [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	25,597	31,514	27,709	35,177
Provision (benefit) for loan losses	(1,332)	1,041	(2,348)	1,099
Charge-offs	(818)	(2,055)	(2,097)	(7,402)
Recoveries	115	261	234	883
Transfers	0	0	0	0
Other	22	115	86	1,119
Ending balance	23,584	30,876	23,584	30,876
Single-family [Member] | Fannie Mae [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	24,490	29,759	26,439	32,956
Provision (benefit) for loan losses	(1,292)	1,268	(2,418)	1,473
Charge-offs	(789)	(2,032)	(2,042)	(7,360)
Recoveries	114	257	231	871
Transfers	191	256	440	615
Other	22	116	86	1,069
Ending balance	22,736	29,624	22,736	29,624
Single-family [Member] | Consolidated Trusts [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	1,107	1,755	1,270	2,221
Provision (benefit) for loan losses	(40)	(227)	70	(374)
Charge-offs	(29)	(23)	(55)	(42)
Recoveries	1	4	3	12
Transfers	(191)	(256)	(440)	(615)
Other	0	(1)	0	50
Ending balance	848	1,252	848	1,252
Multifamily [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	222	306	242	364
Provision (benefit) for loan losses	(4)	(12)	(20)	(62)
Charge-offs	(3)	(19)	(8)	(34)
Recoveries			1	0
Transfers	0	0	0	0
Other	0	(1)	0	6
Ending balance	215	274	215	274
Multifamily [Member] | Fannie Mae [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	67	114	71	161
Provision (benefit) for loan losses	(8)	6	(11)	(31)
Charge-offs	(3)	(19)	(8)	(34)
Recoveries			1	0
Transfers	1	0	4	0
Other	0	(1)	0	4
Ending balance	57	100	57	100
Multifamily [Member] | Consolidated Trusts [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	155	192	171	203
Provision (benefit) for loan losses	4	(18)	(9)	(31)
Charge-offs	0	0	0	0
Recoveries			0	0
Transfers	(1)	0	(4)	0
Other	0	0	0	2
Ending balance	$ 158	$ 174	$ 158	$ 174