Consolidations and Transfers of Financial Assets Unconsolidated VIEs (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Variable Interest Entities [Line Items]
Trading, at fair value	$ 36,886	$ 40,562
Available-for-sale, at fair value	5,963	8,363
Other assets	17,228	20,942
Other liabilities	(8,891)	(10,150)
Unpaid principal balance of mortgage loans	3,131,487	3,063,646
Multifamily [Member]
Variable Interest Entities [Line Items]
Unpaid principal balance of mortgage loans	253,031	229,896
Unconsolidated VIEs [Member] | Multifamily [Member]
Variable Interest Entities [Line Items]
Unpaid principal balance of mortgage loans	253,000
Unconsolidated VIEs [Member] | Mortgage-related securities [Member]
Variable Interest Entities [Line Items]
Trading, at fair value	6,011	8,115
Available-for-sale, at fair value	5,132	7,326
Other assets	73	77
Other liabilities	(481)	(528)
Net carrying amount	10,735	14,990
Maximum exposure to loss	16,000	21,000
Total unconsolidated assets	90,000	150,000
Unconsolidated VIEs [Member] | Partnership interest [Member]
Variable Interest Entities [Line Items]
Net carrying amount	83	92
Maximum exposure to loss	107	118
Total unconsolidated assets	3,200	3,900
Unconsolidated VIEs [Member] | Fannie Mae Securities [Member]
Variable Interest Entities [Line Items]
Trading, at fair value	4,026	4,642
Available-for-sale, at fair value	2,116	2,447
Unconsolidated VIEs [Member] | Non-Fannie Mae Securities [Member]
Variable Interest Entities [Line Items]
Trading, at fair value	1,985	3,473
Available-for-sale, at fair value	$ 3,016	$ 4,879