Contracts Accounted for as Insurance - Loss and LAE Reported on the Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE	$ 37	$ (1)	$ 57	$ 45
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	33	86	92	156
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	33	94	92	164
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	0	(8)	0	(8)
RMBS [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	2	14	8	15
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	4	(88)	(38)	(115)
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	3	4
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	4	(87)	(35)	(111)
Variable Interest Entity, Primary Beneficiary [Member]
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and LAE	$ (2)	$ 14	$ (8)	$ 15