Reinsurance - Credit Quality Information (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	$ 7,195	$ 7,207
Reinsurance Recoverable, Allowance for Credit Loss	(101)	(103)	$ (105)
Net Reinsurance Recoverables	7,094	7,104
Mandatory and Voluntary Risk Pools [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	203	208
Captives [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	353	353
Other not rated companies [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	249	230
AMBestRated [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	6,390	6,416
AM Best, A++ Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	2,394	2,398
AM Best, A+ Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	2,527	2,522
AM Best, A Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	788	811
AM Best, A- Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	652	658
AM Best, B++ Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	7	5
AM Best, Below B plus plus Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	22	22
Operating Segments [Member] | Property, Liability and Casualty Insurance Product Line
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	6,687	6,702
Reinsurance Recoverable, Allowance for Credit Loss	(98)	(100)	(102)	$ (102)
Net Reinsurance Recoverables	6,589	6,602
Operating Segments [Member] | Property, Liability and Casualty Insurance Product Line | Mandatory and Voluntary Risk Pools [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	203	208
Operating Segments [Member] | Property, Liability and Casualty Insurance Product Line | Captives [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	353	353
Operating Segments [Member] | Property, Liability and Casualty Insurance Product Line | Other not rated companies [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	243	226
Operating Segments [Member] | Property, Liability and Casualty Insurance Product Line | Reinsurance Contract [Axis]: Before Disputed Amounts [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverable, Allowance for Credit Loss	(44)	(43)	(44)	(42)
Operating Segments [Member] | Property, Liability and Casualty Insurance Product Line | Reinsurance Contract [Axis]: Disputes [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverable, Allowance for Credit Loss	(54)	(57)	(58)	$ (60)
Operating Segments [Member] | Property, Liability and Casualty Insurance Product Line | AMBestRated [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	5,888	5,915
Operating Segments [Member] | Group Benefits
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	262	261
Reinsurance Recoverable, Allowance for Credit Loss	(1)	(1)	(1)
Net Reinsurance Recoverables	261	260
Operating Segments [Member] | Group Benefits | Mandatory and Voluntary Risk Pools [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	0	0
Operating Segments [Member] | Group Benefits | Captives [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	0	0
Operating Segments [Member] | Group Benefits | Other not rated companies [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	6	4
Operating Segments [Member] | Group Benefits | AMBestRated [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	256	257
Operating Segments [Member] | AM Best, A++ Rating [Member] | Property, Liability and Casualty Insurance Product Line
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	2,394	2,398
Operating Segments [Member] | AM Best, A++ Rating [Member] | Group Benefits
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	0	0
Operating Segments [Member] | AM Best, A+ Rating [Member] | Property, Liability and Casualty Insurance Product Line
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	2,034	2,030
Operating Segments [Member] | AM Best, A+ Rating [Member] | Group Benefits
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	250	251
Operating Segments [Member] | AM Best, A Rating [Member] | Property, Liability and Casualty Insurance Product Line
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	787	810
Operating Segments [Member] | AM Best, A Rating [Member] | Group Benefits
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	1	1
Operating Segments [Member] | AM Best, A- Rating [Member] | Property, Liability and Casualty Insurance Product Line
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	647	653
Operating Segments [Member] | AM Best, A- Rating [Member] | Group Benefits
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	5	5
Operating Segments [Member] | AM Best, B++ Rating [Member] | Property, Liability and Casualty Insurance Product Line
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	4	2
Operating Segments [Member] | AM Best, B++ Rating [Member] | Group Benefits
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	0	0
Operating Segments [Member] | AM Best, Below B plus plus Rating [Member] | Property, Liability and Casualty Insurance Product Line
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	22	22
Operating Segments [Member] | AM Best, Below B plus plus Rating [Member] | Group Benefits
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	0	0
Corporate
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	246	244
Reinsurance Recoverable, Allowance for Credit Loss	(2)	(2)	$ (2)
Net Reinsurance Recoverables	244	242
Corporate | Mandatory and Voluntary Risk Pools [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	0	0
Corporate | Captives [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	0	0
Corporate | Other not rated companies [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	0	0
Corporate | AMBestRated [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	246	244
Corporate | AM Best, A++ Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	0	0
Corporate | AM Best, A+ Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	243	241
Corporate | AM Best, A Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	0	0
Corporate | AM Best, A- Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	0	0
Corporate | AM Best, B++ Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	3	3
Corporate | AM Best, Below B plus plus Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	$ 0	$ 0