Contracts Accounted for as Financial Guaranty Insurance - Loss and LAE Reserve and Salvage and Subrogation Recoverable Net of Reinsurance (Details) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Net Reserve (Recoverable)	$ (124)	$ (139)
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)	(40)	(48)
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)	59	59
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)	(84)	(91)
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)	(64)	(66)
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)	(143)	(150)
Non-U.S. public finance \| 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)	$ 24	$ 18