Insurance Liabilities - Reconciliation of change in net ultimates to prior year development (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	$ (392)
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	(234)
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	(164)
Prior Year Development After Amortization and Re-attribution	(398)
Total, prior years, excluding discount and amortization of deferred gain	(392)
Prior years, excluding discount and amortization of deferred gain	392	$ 530	$ 171
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	(158)
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	(267)	(644)	(617)
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	(114)
Re-Attribution of ADC Recovery	(24)
Amortization of Deferred Gain at Inception	(52)
Prior Year Development After Amortization and Re-attribution	(190)
Total, prior years, excluding discount and amortization of deferred gain	(267)
U.S. Excess Casualty
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(32)	(116)
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	18
Re-Attribution of ADC Recovery	(27)
Amortization of Deferred Gain at Inception	(39)
Prior Year Development After Amortization and Re-attribution	(48)
Total, prior years, excluding discount and amortization of deferred gain	(32)
U.S. Other Casualty
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(133)	(149)
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	(133)
Re-Attribution of ADC Recovery	36
Amortization of Deferred Gain at Inception	(37)
Prior Year Development After Amortization and Re-attribution	(134)
Total, prior years, excluding discount and amortization of deferred gain	(133)
U.S. Financial Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	94	939	649
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	50
Re-Attribution of ADC Recovery	13
Amortization of Deferred Gain at Inception	(26)
Prior Year Development After Amortization and Re-attribution	37
Total, prior years, excluding discount and amortization of deferred gain	94
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	(10)	(81)	172
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	0
Re-Attribution of ADC Recovery	2
Amortization of Deferred Gain at Inception	(9)
Prior Year Development After Amortization and Re-attribution	(7)
Total, prior years, excluding discount and amortization of deferred gain	(10)
U.S. Personal Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(64)		(412)
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	(65)
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	(1)
Prior Year Development After Amortization and Re-attribution	(66)
Total, prior years, excluding discount and amortization of deferred gain	(64)
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	165	82	210
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	165
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Re-attribution	165
Total, prior years, excluding discount and amortization of deferred gain	165
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	81	(153)	(118)
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	81
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Re-attribution	81
Total, prior years, excluding discount and amortization of deferred gain	81
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	(57)	(111)	$ (173)
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	(57)
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Re-attribution	(57)
Total, prior years, excluding discount and amortization of deferred gain	(57)
Other Operations Run-Off
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(7)
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	(7)
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Re-attribution	(7)
Other Product Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(162)	$ (264)
Prior Year Development Net of External Reinsurance After ADC Cessions(a)	(172)
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Re-attribution	$ (172)