Reinsurance and Other Monoline Exposures - Reinsurance and Other Monoline Exposures (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Ceded Credit Risk [Line Items]
Ceded Premium, net of Commissions	$ 42	$ 46
Assumed Expected Loss to be Paid	71	70
Ceded Expected Loss to be Paid	29	87
Assumed par outstanding	8,383	13,264
Second-to pay insured par outstanding	6,605	11,539
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | External Credit Rating, Non Investment Grade [Member]
Ceded Credit Risk [Line Items]
Ceded par outstanding	296	384
Second-to pay insured par outstanding	$ 204	$ 788