Contracts Accounted for as Insurance - Loss and LAE Reserve and Salvage and Subrogation Recoverable Net of Reinsurance (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Net Reserve (Recoverable)	$ (153)	$ 122
Variable Interest Entity, Primary Beneficiary
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Net Reserve (Recoverable)	19	32
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)	(155)	140
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)	36	34
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)	2	(18)
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)	(156)	129
United States | 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)	(34)	(52)
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)	$ 1	$ 11