Financial Guarantees Financial Guarantees and Maximum Recovery (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Financial Guarantees and Maximum Recovery [Line Items]
Maximum Exposure	$ 21,904	$ 25,141
Guaranty Obligation	169	258
Maximum Recovery	9,557	9,744
Fair value of guaranty obligations in Investments in securities	8	276
Unconsolidated VIEs [Member] | Unconsolidated Fannie Mae MBS [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Maximum Exposure	7,944	10,876
Guaranty Obligation	31	127
Maximum Recovery	7,201	7,340
Unconsolidated VIEs [Member] | Other guaranty arrangements [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Maximum Exposure	13,960	14,265
Guaranty Obligation	138	131
Maximum Recovery	$ 2,356	$ 2,404
Minimum [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Contractual terms of our guarantees1	8 days
Maximum [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Contractual terms of our guarantees1	34 years