Reinsurance - Ceded Reinsurance (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Due (To) From [Abstract]
Ceded premium payable, net of commissions	$ 20	$ 26
Ceded expected loss to be recovered (paid)	11	14
Financial Guaranty [Abstract]
Financial guaranty Ceded par outstanding	1,349	2,389
Non-Financial Guaranty Exposure [Abstract]
Specialty ceded exposure (see Note 5)	303	239
Non-affiliated Reinsurers [Member]
Non-Financial Guaranty Exposure [Abstract]
Collateral posted by non-affiliated reinsurers	68	80
Below Investment Grade, Rating Withdrawn Or Not Rated Reinsurer [Member] | External Credit Rating, Non Investment Grade [Member]
Non-Financial Guaranty Exposure [Abstract]
Ceded par outstanding	$ 224	$ 236