Insurance Liabilities - Reconciliation of Net Prior Year Development (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	$ 254
Prior Year Development Net of External Reinsurance After ADC Cessions	(120)
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	(136)
Prior Year Development After Amortization and Re-attribution	(256)
Total, prior years, excluding discount and amortization of deferred gain	254
Asbestos Loss Portfolio Transfer
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year development ceded	85
Adverse Development Reinsurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year development ceded	289
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	(273)	$ (267)	$ (644)
Prior Year Development Net of External Reinsurance After ADC Cessions	(205)
Re-Attribution of ADC Recovery	(14)
Amortization of Deferred Gain at Inception	(42)
Prior Year Development After Amortization and Re-attribution	(261)
Total, prior years, excluding discount and amortization of deferred gain	(273)
U.S. Excess Casualty
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	545	(32)	(116)
Prior Year Development Net of External Reinsurance After ADC Cessions	238
Re-Attribution of ADC Recovery	22
Amortization of Deferred Gain at Inception	(32)
Prior Year Development After Amortization and Re-attribution	228
Total, prior years, excluding discount and amortization of deferred gain	545
U.S. Other Casualty
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	12	(133)	(149)
Prior Year Development Net of External Reinsurance After ADC Cessions	44
Re-Attribution of ADC Recovery	(38)
Amortization of Deferred Gain at Inception	(31)
Prior Year Development After Amortization and Re-attribution	(25)
Total, prior years, excluding discount and amortization of deferred gain	12
U.S. Financial Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	51	94	939
Prior Year Development Net of External Reinsurance After ADC Cessions	(40)
Re-Attribution of ADC Recovery	20
Amortization of Deferred Gain at Inception	(23)
Prior Year Development After Amortization and Re-attribution	(43)
Total, prior years, excluding discount and amortization of deferred gain	51
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	(44)	(10)	(81)
Prior Year Development Net of External Reinsurance After ADC Cessions	4
Re-Attribution of ADC Recovery	11
Amortization of Deferred Gain at Inception	(7)
Prior Year Development After Amortization and Re-attribution	8
Total, prior years, excluding discount and amortization of deferred gain	(44)
U.S. Personal Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(23)	(64)
Prior Year Development Net of External Reinsurance After ADC Cessions	(25)
Re-Attribution of ADC Recovery	(1)
Amortization of Deferred Gain at Inception	(1)
Prior Year Development After Amortization and Re-attribution	(27)
Total, prior years, excluding discount and amortization of deferred gain	(23)
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	170	165	82
Prior Year Development Net of External Reinsurance After ADC Cessions	170
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Re-attribution	170
Total, prior years, excluding discount and amortization of deferred gain	170
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	(35)	81	(153)
Prior Year Development Net of External Reinsurance After ADC Cessions	(35)
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Re-attribution	(35)
Total, prior years, excluding discount and amortization of deferred gain	(35)
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	(47)	(57)	(111)
Prior Year Development Net of External Reinsurance After ADC Cessions	(47)
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Re-attribution	(47)
Total, prior years, excluding discount and amortization of deferred gain	(47)
Other Operations
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	1
Prior Year Development Net of External Reinsurance After ADC Cessions	1
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Re-attribution	1
Other product lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(299)	$ (162)	$ (264)
Prior Year Development Net of External Reinsurance After ADC Cessions	(336)
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Re-attribution	(336)
Subtotal, adjusted pre-tax basis
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	58
Prior Year Development Net of External Reinsurance After ADC Cessions	(231)
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	(136)
Prior Year Development After Amortization and Re-attribution	(367)
Businesses in run-off
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	196
Prior Year Development Net of External Reinsurance After ADC Cessions	111
Re-Attribution of ADC Recovery	0
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Re-attribution	111
Businesses in run-off | Asbestos Loss Portfolio Transfer
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	$ 85