Financial Guarantees Financial Guarantees and Maximum Recovery (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Financial Guarantees and Maximum Recovery [Line Items]
Maximum Exposure	$ 21,509	$ 25,141
Guaranty Obligation	162	258
Maximum Recovery	9,329	9,744
Unconsolidated VIEs [Member] | Unconsolidated Fannie Mae MBS [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Maximum Exposure	7,485	10,876
Guaranty Obligation	31	127
Maximum Recovery	6,930	7,340
Unconsolidated VIEs [Member] | Other guaranty arrangements [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Maximum Exposure	14,024	14,265
Guaranty Obligation	131	131
Maximum Recovery	$ 2,399	$ 2,404
Minimum [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Contractual terms of our guarantees	1 day
Maximum [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Contractual terms of our guarantees	34 years