Reinsurance and Other Monoline Exposures - Exposure by Reinsurer (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Ceded Credit Risk [Line Items]
Assumed Premiums, Net of Commissions	$ 53	$ 65
Assumed	10	(11)	$ 17
Ceded premium, net of commissions	(42)	(46)
Assumed expected loss to be paid	(71)	(70)
Ceded expected loss to be paid	29	87
Insured Financial Obligations, Ceded Par Outstanding	4,434	11,156
Assumed par outstanding	8,383	13,264
Second-to pay insured par outstanding	6,605	11,539
Non-affiliated Reinsurers [Member]
Ceded Credit Risk [Line Items]
Collateral posted by non-affiliated reinsurers	118	387
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | External Credit Rating, Non Investment Grade [Member]
Ceded Credit Risk [Line Items]
Second-to pay insured par outstanding	204	788
Ceded par outstanding	$ 296	$ 384