Contracts Accounted for as Insurance - Loss and LAE Reserve and Salvage and Subrogation Recoverable Net of Reinsurance (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Net Reserve (Recoverable)	$ 47	$ 295
Variable Interest Entity, Primary Beneficiary
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Net Reserve (Recoverable)	35	33
Public finance | Financial Guarantee Insurance And Other Product Line
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Net Reserve (Recoverable)	74	333
Other structured finance | Financial Guarantee Insurance And Other Product Line
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Net Reserve (Recoverable)	42	40
Structured finance | Financial Guarantee Insurance And Other Product Line
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Net Reserve (Recoverable)	(27)	(38)
U.S. | Public finance | Financial Guarantee Insurance And Other Product Line
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Net Reserve (Recoverable)	66	328
U.S. | RMBS | Financial Guarantee Insurance And Other Product Line
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Net Reserve (Recoverable)	(69)	(78)
Non United States | Public finance | Financial Guarantee Insurance And Other Product Line
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Net Reserve (Recoverable)	$ 8	$ 5