Allowance for Loan Losses Rollforward by Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Allowance for Loan Losses [Roll Forward]
Beginning balance	$ 23,799	$ 31,150	$ 27,951	$ 35,541
Provision (benefit) for loan losses	(636)	(1,394)	(3,004)	(357)
Charge-offs	(608)	(777)	(2,713)	(8,213)
Recoveries	141	168	376	1,051
Transfers	0	0	0	0
Other	10	(12)	96	1,113
Ending balance	22,706	29,135	22,706	29,135
Fannie Mae [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	22,793	29,724	26,510	33,117
Provision (benefit) for loan losses	(583)	(1,732)	(3,012)	(290)
Charge-offs	(589)	(753)	(2,639)	(8,147)
Recoveries	114	165	346	1,036
Transfers	113	263	557	878
Other	10	(12)	96	1,061
Ending balance	21,858	27,655	21,858	27,655
Consolidated Trusts [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	1,006	1,426	1,441	2,424
Provision (benefit) for loan losses	(53)	338	8	(67)
Charge-offs	(19)	(24)	(74)	(66)
Recoveries	27	3	30	15
Transfers	(113)	(263)	(557)	(878)
Other	0	0	0	52
Ending balance	848	1,480	848	1,480
Single-family [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	23,584	30,876	27,709	35,177
Provision (benefit) for loan losses	(609)	(1,392)	(2,957)	(293)
Charge-offs	(604)	(770)	(2,701)	(8,172)
Recoveries	135	164	369	1,047
Transfers	0	0	0	0
Other	10	(13)	96	1,106
Ending balance	22,516	28,865	22,516	28,865
Single-family [Member] | Fannie Mae [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	22,736	29,624	26,439	32,956
Provision (benefit) for loan losses	(561)	(1,722)	(2,979)	(249)
Charge-offs	(587)	(748)	(2,629)	(8,108)
Recoveries	108	161	339	1,032
Transfers	113	262	553	877
Other	10	(13)	96	1,056
Ending balance	21,819	27,564	21,819	27,564
Single-family [Member] | Consolidated Trusts [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	848	1,252	1,270	2,221
Provision (benefit) for loan losses	(48)	330	22	(44)
Charge-offs	(17)	(22)	(72)	(64)
Recoveries	27	3	30	15
Transfers	(113)	(262)	(553)	(877)
Other	0	0	0	50
Ending balance	697	1,301	697	1,301
Multifamily [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	215	274	242	364
Provision (benefit) for loan losses	(27)	(2)	(47)	(64)
Charge-offs	(4)	(7)	(12)	(41)
Recoveries	6	4	7	4
Transfers	0	0	0	0
Other	0	1	0	7
Ending balance	190	270	190	270
Multifamily [Member] | Fannie Mae [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	57	100	71	161
Provision (benefit) for loan losses	(22)	(10)	(33)	(41)
Charge-offs	(2)	(5)	(10)	(39)
Recoveries	6	4	7	4
Transfers	0	1	4	1
Other	0	1	0	5
Ending balance	39	91	39	91
Multifamily [Member] | Consolidated Trusts [Member]
Allowance for Loan Losses [Roll Forward]
Beginning balance	158	174	171	203
Provision (benefit) for loan losses	(5)	8	(14)	(23)
Charge-offs	(2)	(2)	(2)	(2)
Recoveries	0	0	0	0
Transfers	0	(1)	(4)	(1)
Other	0	0	0	2
Ending balance	$ 151	$ 179	$ 151	$ 179