Consolidations and Transfers of Financial Assets Unconsolidated VIEs (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Variable Interest Entities [Line Items]
Trading, at fair value	$ 42,381	$ 34,679
Available-for-sale, at fair value	3,723	4,843
Other assets	17,160	19,049
Other liabilities	(8,591)	(9,479)
Unpaid principal balance of mortgage loans	3,186,600	3,155,703
Multifamily [Member]
Variable Interest Entities [Line Items]
Unpaid principal balance of mortgage loans	275,797	265,069
Unconsolidated VIEs [Member] | Mortgage-related securities [Member]
Variable Interest Entities [Line Items]
Trading, at fair value	6,565	5,389
Available-for-sale, at fair value	3,147	4,094
Other assets	70	74
Other liabilities	(104)	(467)
Net carrying amount	9,678	9,090
Maximum exposure to loss	14,000	15,000
Total unconsolidated assets	70,000	70,000
Unconsolidated VIEs [Member] | Partnership interest [Member]
Variable Interest Entities [Line Items]
Net carrying amount	81	82
Maximum exposure to loss	103	105
Total unconsolidated assets	3,000	3,200
Unconsolidated VIEs [Member] | Fannie Mae Securities [Member]
Variable Interest Entities [Line Items]
Trading, at fair value	1,640	3,809
Available-for-sale, at fair value	1,825	2,032
Unconsolidated VIEs [Member] | Non-Fannie Mae Securities [Member]
Variable Interest Entities [Line Items]
Trading, at fair value	4,925	1,580
Available-for-sale, at fair value	$ 1,322	$ 2,062