Contracts Accounted for as Insurance - Loss and LAE Reported on the Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE	$ (17)	$ (223)	$ (43)	$ (354)
Variable Interest Entity, Primary Beneficiary [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE	(3)	(1)	0	(5)
Public Finance [Member] | Financial Guarantee Insurance And Other Product Line [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE	(39)	(233)	(71)	(421)
Public Finance [Member] | Financial Guarantee Insurance And Other Product Line [Member] | United States [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE	(42)	(233)	(76)	(424)
Public Finance [Member] | Financial Guarantee Insurance And Other Product Line [Member] | Non United States [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE	3	0	5	3
RMBS [Member] | Financial Guarantee Insurance And Other Product Line [Member] | United States [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE	21	5	17	19
Other structured finance [Member] | Financial Guarantee Insurance And Other Product Line [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE	1	5	11	48
Structured Finance [Member] | Financial Guarantee Insurance And Other Product Line [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE	$ 22	$ 10	$ 28	$ 67