Contracts Accounted for as Insurance - Loss and LAE Reported on the Statements of Operations (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and loss adjustment expenses (benefit)	$ 28	$ (2)	$ 68	$ (3)
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)	29	1	71	(1)
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)	15	1	51	(1)
Public finance \| Financial Guarantee Insurance And Other Product Line \| Non United States
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items]
Loss and loss adjustment expenses (benefit)	14	0	20	0
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)	0	(5)	0	(3)
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	(3)	1
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)	$ (3)	$ (3)	$ (2)