Consolidations and Transfers of Financial Assets Unconsolidated VIEs (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Variable Interest Entities [Line Items]
Trading, at fair value	$ 40,097	$ 34,679
Available-for-sale, at fair value	3,888	4,843
Other assets	17,933	19,049
Other liabilities	(10,219)	(9,479)
Unpaid principal balance of mortgage loans	3,176,703	3,155,703
Multifamily [Member]
Variable Interest Entities [Line Items]
Unpaid principal balance of mortgage loans	269,316	265,069
Unconsolidated VIEs [Member] | Multifamily [Member]
Variable Interest Entities [Line Items]
Unpaid principal balance of mortgage loans	269,300
Unconsolidated VIEs [Member] | Mortgage-related securities [Member]
Variable Interest Entities [Line Items]
Trading, at fair value	6,779	5,389
Available-for-sale, at fair value	3,277	4,094
Other assets	73	74
Other liabilities	(114)	(467)
Net carrying amount	10,015	9,090
Maximum exposure to loss	15,000	15,000
Total unconsolidated assets	70,000	70,000
Unconsolidated VIEs [Member] | Partnership interest [Member]
Variable Interest Entities [Line Items]
Net carrying amount	80	82
Maximum exposure to loss	102	105
Total unconsolidated assets	3,200	3,200
Unconsolidated VIEs [Member] | Fannie Mae Securities [Member]
Variable Interest Entities [Line Items]
Trading, at fair value	1,679	3,809
Available-for-sale, at fair value	1,904	2,032
Unconsolidated VIEs [Member] | Non-Fannie Mae Securities [Member]
Variable Interest Entities [Line Items]
Trading, at fair value	5,100	1,580
Available-for-sale, at fair value	$ 1,373	$ 2,062