Guarantees Repurchase Requests (Additional Information) (Details) - USD ($) $ in Millions	Mar. 31, 2017		Dec. 31, 2016	Mar. 31, 2016		Dec. 31, 2015
Unpaid Principal Balance Of Unresolved Repurchase Requests	$ 6	[1]	$ 14	$ 14	[1]	$ 17
US Government Sponsored Agency [Member]
Unpaid Principal Balance Of Unresolved Repurchase Requests	5			9
Non-Government Sponsored Agency [Member]
Unpaid Principal Balance Of Unresolved Repurchase Requests	$ 1			$ 5

[1] Comprised of $5 million and $9 million from the GSEs, and $1 million and $5 million from non-agency investors at March 31, 2017 and 2016, respectively.