Consolidations and Transfers of Financial Assets Unconsolidated VIEs (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Variable Interest Entities [Line Items]
Trading, at fair value	$ 43,901	$ 34,679
Available-for-sale, at fair value	3,537	4,843
Other assets	17,022	19,049
Other liabilities	(9,655)	(9,479)
Unpaid principal balance of mortgage loans	3,209,155	3,155,703
Multifamily [Member]
Variable Interest Entities [Line Items]
Unpaid principal balance of mortgage loans	284,298	265,069
Unconsolidated VIEs [Member] | Mortgage-related securities [Member]
Variable Interest Entities [Line Items]
Trading, at fair value	6,429	5,389
Available-for-sale, at fair value	3,008	4,094
Other assets	69	74
Other liabilities	(108)	(467)
Net carrying amount	9,398	9,090
Maximum exposure to loss	14,000	15,000
Total unconsolidated assets	90,000	70,000
Unconsolidated VIEs [Member] | Partnership interest [Member]
Variable Interest Entities [Line Items]
Net carrying amount	83	82
Maximum exposure to loss	105	105
Total unconsolidated assets	2,400	3,200
Unconsolidated VIEs [Member] | Fannie Mae Securities [Member]
Variable Interest Entities [Line Items]
Trading, at fair value	1,420	3,809
Available-for-sale, at fair value	1,754	2,032
Unconsolidated VIEs [Member] | Non-Fannie Mae Securities [Member]
Variable Interest Entities [Line Items]
Trading, at fair value	5,009	1,580
Available-for-sale, at fair value	$ 1,254	$ 2,062