Insurance Contracts Financial Guarantee Insurance Contracts - Components of Loss and Loss Expense Reserves and Subrogation Recoverable (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Dec. 31, 2022	Dec. 31, 2021
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered		$ 1
Policyholder Benefits and Claims Incurred, Ceded	$ 75	30
Subrogation recoverable	(179)		$ (271)
Claim liability reported on Balance Sheet, before reinsurance	510		444
Liability for Claims and Claims Adjustment Expense	850		805
Liability for Claim and Claim Adjustment Expenses Net of Subrogation recoverables	670	$ (940)	534	$ (522)
Property and Casualty, Commercial Insurance
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Liability for Claims and Claims Adjustment Expense	160		90
Liability for Claim and Claim Adjustment Expenses Net of Subrogation recoverables	160		90
Present Value of Expected Net Cash Flows- Claims and Loss Expenses [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Subrogation recoverable	4		5
Claim liability reported on Balance Sheet, before reinsurance	773		791
Liability for Claims and Claims Adjustment Expense	769		787
Present Value of Expected Net Cash Flows-Recoveries [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Subrogation recoverable	(183)		(276)
Claim liability reported on Balance Sheet, before reinsurance	(234)		(319)
Liability for Claims and Claims Adjustment Expense	(50)		(44)
Unearned Premium Reserve [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Subrogation recoverable	0		0
Claim liability reported on Balance Sheet, before reinsurance	(29)		(28)
Liability for Claims and Claims Adjustment Expense	$ (29)		$ (28)