Insurance Contracts Financial Guarantee Insurance Contracts - Components of Loss and Loss Expense Reserves and Subrogation Recoverable (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Dec. 31, 2022
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Ceded Loss And Loss Expenses Paid Not Yet Recovered		$ 3
Policyholder Benefits and Claims Incurred, Ceded	$ 35	20
Subrogation recoverable	(130)		$ (137)
Claim liability reported on Balance Sheet, before reinsurance	484		559
Liability for Claims and Claims Adjustment Expense	851		893
Liability for Claim and Claim Adjustment Expenses Net of Subrogation recoverables	721	$ 705	756	$ 534
Property and Casualty, Commercial Insurance
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Liability for Claims and Claims Adjustment Expense	237		197
Liability for Claim and Claim Adjustment Expenses Net of Subrogation recoverables	237		197
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	1		1
Claim liability reported on Balance Sheet, before reinsurance	695		780
Liability for Claims and Claims Adjustment Expense	693		779
Present Value of Expected Net Cash Flows-Recoveries [Member]
Components of Loss and Loss Expense Reserves and Subrogation Recoverable [Line Items]
Subrogation recoverable	(131)		(139)
Claim liability reported on Balance Sheet, before reinsurance	(187)		(194)
Liability for Claims and Claims Adjustment Expense	(56)		(55)
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	(24)		(28)
Liability for Claims and Claims Adjustment Expense	$ (24)		$ (28)