Contracts Accounted for as Financial Guaranty Insurance - Loss and LAE Reported on the Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and loss adjustment expenses (benefit)	$ 17	$ 40
Public finance \| Financial Guarantee Insurance And Other Product Line
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and loss adjustment expenses (benefit)	19	42
Public finance \| Financial Guarantee Insurance And Other Product Line \| United States
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and loss adjustment expenses (benefit)	12	36
Public finance \| Financial Guarantee Insurance And Other Product Line \| Non-U.S. public finance
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and loss adjustment expenses (benefit)	7	6
RMBS \| Financial Guarantee Insurance And Other Product Line \| United States
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and loss adjustment expenses (benefit)	(1)	0
Other structured finance \| Financial Guarantee Insurance And Other Product Line
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and loss adjustment expenses (benefit)	(1)	(2)
Structured finance \| Financial Guarantee Insurance And Other Product Line
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and loss adjustment expenses (benefit)	$ (2)	$ (2)