Basis of Presentation and Significant Accounting Policies (Details) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Participating dividends to policyholders	$ 39,000,000	$ 29,000,000	$ 24,000,000
Derivative Liability, Statement of Financial Position [Extensible Enumeration]	Other investments, Other liabilities	Other investments, Other liabilities
Deposit Contracts, Liabilities	$ 0	$ 0
Accumulated depreciation	2,400,000,000	2,300,000,000
Depreciation expense	204,000,000	213,000,000	282,000,000
Hosting Arrangement, Service Contract, Implementation Cost, Expense, Amortization	$ 85,000,000	78,000,000	60,000,000
Minimum [Member] | Other Intangible Assets [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Finite-Lived Intangible Asset, Useful Life	1 year
Maximum | Other Intangible Assets [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Finite-Lived Intangible Asset, Useful Life	15 years
Reinsurance Policy, Type [Axis]: Adverse Development Cover Navigators Group
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Revenue	$ 209,000,000
Reinsurance Policy, Type [Axis]: Retroactive Reinsurance [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Paid loss and loss adjustment expenses	1,700,000,000	1,500,000,000
Deferred Revenue, Additions	194,000,000	229,000,000	$ 246,000,000
Reinsurance Policy, Type [Axis]: Retroactive Reinsurance [Member] | Other liabilities
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Revenue	$ 997,000,000	$ 803,000,000
Property and Casualty Insurance Subsidiaries
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Participating Insurance, Percentage of Premium Income	6.00%	7.00%	7.00%