Reinsurance - Credit Quality Information (Details) - USD ($) $ in Millions	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	$ 6,749		$ 6,622
Reinsurance Recoverable, Allowance for Credit Loss	(105)	$ (105)	(99)	$ (99)	$ (99)	$ (108)
Net Reinsurance Recoverables	6,644		6,523
Mandatory and Voluntary Risk Pools [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	227		239
Captives [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	347		331
Other not rated companies [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	258		260
Reinsurance Contract [Axis]: Adverse Development Cover Navigators Group
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	300
Reinsurance Contract [Axis]: Before Uncollectible Reinsurance [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverable, Allowance for Credit Loss	(45)	(44)	(45)	(44)	(43)	(55)
Reinsurance Contract [Axis]: Disputes [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverable, Allowance for Credit Loss	(60)	(61)	(54)	(55)	(56)	(53)
AMBestRated [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	5,917		5,792
AM Best, A++ Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	1,867		1,860
AM Best, A+ Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	2,613		2,511
AM Best, A Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	743		713
AM Best, A- Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	51		46
AM Best, B++ Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	623		642
AM Best, Below B plus plus Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	20		20
Corporate
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	269		278
Reinsurance Recoverable, Allowance for Credit Loss	(2)	(2)	(2)	(2)	(2)	(2)
Net Reinsurance Recoverables	267		276
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 | Reinsurance Contract [Axis]: Before Uncollectible Reinsurance [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverable, Allowance for Credit Loss	(2)	(2)	(2)	(2)	(2)	(2)
Corporate | Reinsurance Contract [Axis]: Disputes [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverable, Allowance for Credit Loss	0	0	0	0	0	0
Corporate | AMBestRated [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	269		278
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	267		275
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	2		3
Corporate | AM Best, Below B plus plus Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	0		0
Property, Liability and Casualty Insurance Product Line | Operating Segments [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	6,220		6,093
Reinsurance Recoverable, Allowance for Credit Loss	(102)	(102)	(96)	(96)	(96)	(105)
Net Reinsurance Recoverables	6,118		5,997
Property, Liability and Casualty Insurance Product Line | Operating Segments [Member] | Mandatory and Voluntary Risk Pools [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	227		239
Property, Liability and Casualty Insurance Product Line | Operating Segments [Member] | Captives [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	347		331
Property, Liability and Casualty Insurance Product Line | Operating Segments [Member] | Other not rated companies [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	251		255
Property, Liability and Casualty Insurance Product Line | Operating Segments [Member] | Reinsurance Contract [Axis]: Before Uncollectible Reinsurance [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverable, Allowance for Credit Loss	(42)	(41)	(42)	(41)	(40)	(52)
Property, Liability and Casualty Insurance Product Line | Operating Segments [Member] | Reinsurance Contract [Axis]: Disputes [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverable, Allowance for Credit Loss	(60)	(61)	(54)	(55)	(56)	(53)
Property, Liability and Casualty Insurance Product Line | Operating Segments [Member] | AMBestRated [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	5,395		5,268
Property, Liability and Casualty Insurance Product Line | Operating Segments [Member] | AM Best, A++ Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	1,867		1,860
Property, Liability and Casualty Insurance Product Line | Operating Segments [Member] | AM Best, A+ Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	2,102		1,999
Property, Liability and Casualty Insurance Product Line | Operating Segments [Member] | AM Best, A Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	742		713
Property, Liability and Casualty Insurance Product Line | Operating Segments [Member] | AM Best, A- Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	43		37
Property, Liability and Casualty Insurance Product Line | Operating Segments [Member] | AM Best, B++ Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	621		639
Property, Liability and Casualty Insurance Product Line | Operating Segments [Member] | AM Best, Below B plus plus Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	20		20
Group Benefits | Operating Segments [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	260		251
Reinsurance Recoverable, Allowance for Credit Loss	(1)	(1)	(1)	(1)	(1)	(1)
Net Reinsurance Recoverables	259		250
Group Benefits | Operating Segments [Member] | Mandatory and Voluntary Risk Pools [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	0		0
Group Benefits | Operating Segments [Member] | Captives [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	0		0
Group Benefits | Operating Segments [Member] | Other not rated companies [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	7		5
Group Benefits | Operating Segments [Member] | Reinsurance Contract [Axis]: Before Uncollectible Reinsurance [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverable, Allowance for Credit Loss	(1)	(1)	(1)	(1)	(1)	(1)
Group Benefits | Operating Segments [Member] | Reinsurance Contract [Axis]: Disputes [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverable, Allowance for Credit Loss	0	$ 0	0	$ 0	$ 0	$ 0
Group Benefits | Operating Segments [Member] | AMBestRated [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	253		246
Group Benefits | Operating Segments [Member] | AM Best, A++ Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	0		0
Group Benefits | Operating Segments [Member] | AM Best, A+ Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	244		237
Group Benefits | Operating Segments [Member] | AM Best, A Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	1		0
Group Benefits | Operating Segments [Member] | AM Best, A- Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	8		9
Group Benefits | Operating Segments [Member] | AM Best, B++ Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	0		0
Group Benefits | Operating Segments [Member] | AM Best, Below B plus plus Rating [Member]
Reinsurance Recoverable, Credit Quality Indicator [Line Items]
Reinsurance Recoverables, Gross	$ 0		$ 0