Financial Guarantees and Maximum Recovery (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Financial Guarantees and Maximum Recovery [Line Items]
Maximum exposure	$ 25,141	$ 27,942
Guaranty obligation	258	280
Maximum recovery	9,744	10,711
Unconsolidated VIEs [Member] | Unconsolidated Fannie Mae MBS [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Maximum exposure	10,876	12,607
Guaranty obligation	127	143
Maximum recovery	7,340	8,048
Unconsolidated VIEs [Member] | Other guaranty arrangements [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Maximum exposure	14,265	15,335
Guaranty obligation	131	137
Maximum recovery	$ 2,404	$ 2,663
Minimum [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Contractual terms of our guarantees	5 days
Maximum [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Contractual terms of our guarantees	35 years