Financial Guarantees Financial Guarantees and Maximum Recovery (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Financial Guarantees and Maximum Recovery [Line Items]
Maximum Exposure	$ 17,886	$ 18,471
Guaranty Obligation	141	154
Maximum Recovery	7,739	8,098
Unconsolidated VIEs [Member] | Unconsolidated Fannie Mae MBS [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Maximum Exposure	5,487	5,801
Guaranty Obligation	18	26
Maximum Recovery	5,244	5,545
Unconsolidated VIEs [Member] | Other guaranty arrangements [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Maximum Exposure	12,399	12,670
Guaranty Obligation	123	128
Maximum Recovery	2,495	2,553
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] | Unconsolidated VIEs [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Single security, Freddie Mac issued securities backing unconsolidated Fannie Mae securities	$ 72,700	$ 50,100
Maximum [Member]
Financial Guarantees and Maximum Recovery [Line Items]
Contractual terms of our guarantees	33 years