Guarantees Repurchase Requests (Additional Information) (Details) - USD ($) $ in Millions	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014		Dec. 31, 2013
Unpaid Principal Balance Of Unresolved Repurchase Requests	$ 14	[1]	$ 17	[1]	$ 47	[1]	$ 126
US Government Sponsored Agency [Member]
Unpaid Principal Balance Of Unresolved Repurchase Requests	8		11		44
Non-Government Sponsored Agency [Member]
Unpaid Principal Balance Of Unresolved Repurchase Requests	$ 6		$ 6		$ 3

[1] Comprised of $8 million, $11 million, and $44 million from the GSEs, and $6 million, $6 million, and $3 million from non-agency investors at December 31, 2016, 2015, and 2014 respectively.