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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE	$ 30	$ 17	$ 75	$ 43
Variable Interest Entity, Primary Beneficiary [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE	3	3	18	0
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	65	39	221	71
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	64	42	228	76
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	1	(3)	(7)	(5)
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	(35)	(21)	(150)	(17)
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	0	(1)	4	(11)
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	$ (35)	$ (22)	$ (146)	$ (28)