Guarantees Repurchase Requests (Additional Information) (Details) - USD ($) $ in Millions	Sep. 30, 2017		Dec. 31, 2016	Sep. 30, 2016		Dec. 31, 2015
Unpaid Principal Balance Of Unresolved Repurchase Requests	$ 9	[1]	$ 14	$ 9	[1]	$ 17
US Government Sponsored Agency [Member]
Unpaid Principal Balance Of Unresolved Repurchase Requests	9			4
Non-Government Sponsored Agency [Member]
Unpaid Principal Balance Of Unresolved Repurchase Requests	$ 1			$ 4

[1] Comprised of $9 million and $4 million from the GSEs, and less than $1 million and $4 million from non-agency investors at September 30, 2017 and 2016, respectively.