Contracts Accounted for as Financial Guaranty Insurance - Loss and LAE Reported on the Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and loss adjustment expenses (benefit)	$ 4	$ 28	$ 21	$ 68
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)	1	29	20	71
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)	(3)	15	9	51
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)	4	14	11	20
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)	2	0	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	(1)	0	(3)
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)	$ 3	$ (1)	$ 1	$ (3)