Insurance Liabilities - Reconciliation of change in net ultimates to prior year development (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	$ (530)
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	(356)
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	(167)
Prior Year Development After Amortization and Re-attribution	(523)
Total, prior years, excluding discount and amortization of deferred gain	(530)
Prior years, excluding discount and amortization of deferred gain	530	$ 171	$ 90
Asbestos Loss Portfolio Transfer
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year development ceded	0
Adverse Development Reinsurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year development ceded	(174)
U.S. Workers' Compensation (before discount)
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(644)	(617)	(367)
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	(338)
Re-Attribution of ADC Recovery	(29)
Amortization of Deferred Gain at Inception	(52)
Prior Year Development After Amortization and Re-attribution	(419)
Total, prior years, excluding discount and amortization of deferred gain	(644)
U.S. Excess Casualty
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(116)		(149)
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	17
Re-Attribution of ADC Recovery	15
Amortization of Deferred Gain at Inception	(40)
Prior Year Development After Amortization and Re-attribution	(8)
Total, prior years, excluding discount and amortization of deferred gain	(116)
U.S. Other Casualty
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(149)		(141)
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	(162)
Re-Attribution of ADC Recovery	33
Amortization of Deferred Gain at Inception	(38)
Prior Year Development After Amortization and Re-attribution	(167)
Total, prior years, excluding discount and amortization of deferred gain	(149)
U.S. Financial Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	939	649	479
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	707
Re-Attribution of ADC Recovery	(22)
Amortization of Deferred Gain at Inception	(27)
Prior Year Development After Amortization and Re-attribution	658
Total, prior years, excluding discount and amortization of deferred gain	939
U.S. Property and Special Risks
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(81)	172	(80)
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	(100)
Re-Attribution of ADC Recovery	3
Amortization of Deferred Gain at Inception	(9)
Prior Year Development After Amortization and Re-attribution	(106)
Total, prior years, excluding discount and amortization of deferred gain	(81)
U.S. Personal Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(28)	(412)	94
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	(32)
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	(1)
Prior Year Development After Amortization and Re-attribution	(33)
Total, prior years, excluding discount and amortization of deferred gain	(28)
UK/Europe Casualty and Financial lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	82	210	258
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	82
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Re-attribution	82
Total, prior years, excluding discount and amortization of deferred gain	82
UK/Europe Property and Special Risks
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(153)	(118)	(155)
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	(153)
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Re-attribution	(153)
Total, prior years, excluding discount and amortization of deferred gain	(153)
UK/Europe and Japan Personal Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(111)	$ (173)	$ (39)
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	(111)
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Re-attribution	(111)
Total, prior years, excluding discount and amortization of deferred gain	(111)
Other Operations Run-Off
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(5)
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	(5)
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Re-attribution	(5)
Other Product Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(264)
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	(261)
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Re-attribution	$ (261)