Insurance Liabilities - Reconciliation of Net Prior Year Development (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	$ (216)
Prior Year Development Net of External Reinsurance After ADC Cessions	(427)
Reattribution of ADC Recovery and Other	0
Amortization of Deferred Gain at Inception	(124)
Prior Year Development After Amortization and Reattribution	(551)
Total, prior years, excluding discount and amortization of deferred gain	(216)
Asbestos Loss Portfolio Transfer
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year development ceded	109
Adverse Development Reinsurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior year development ceded	102
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	(133)	$ (273)	$ (267)
Prior Year Development Net of External Reinsurance After ADC Cessions	(113)
Reattribution of ADC Recovery and Other	(21)
Amortization of Deferred Gain at Inception	(38)
Prior Year Development After Amortization and Reattribution	(172)
U.S. Excess Casualty
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	303	545	(32)
Prior Year Development Net of External Reinsurance After ADC Cessions	82
Reattribution of ADC Recovery and Other	33
Amortization of Deferred Gain at Inception	(30)
Prior Year Development After Amortization and Reattribution	85
U.S. Other Casualty
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(92)	12	(133)
Prior Year Development Net of External Reinsurance After ADC Cessions	22
Reattribution of ADC Recovery and Other	(2)
Amortization of Deferred Gain at Inception	(28)
Prior Year Development After Amortization and Reattribution	(8)
U.S. Financial Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(38)	51	94
Prior Year Development Net of External Reinsurance After ADC Cessions	(45)
Reattribution of ADC Recovery and Other	0
Amortization of Deferred Gain at Inception	(20)
Prior Year Development After Amortization and Reattribution	(65)
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	(118)	(44)	(10)
Prior Year Development Net of External Reinsurance After ADC Cessions	(150)
Reattribution of ADC Recovery and Other	26
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Reattribution	(124)
U.S. Personal Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(11)	(23)	(64)
Prior Year Development Net of External Reinsurance After ADC Cessions	(11)
Reattribution of ADC Recovery and Other	2
Amortization of Deferred Gain at Inception	(1)
Prior Year Development After Amortization and Reattribution	(10)
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	216	170	165
Prior Year Development Net of External Reinsurance After ADC Cessions	216
Reattribution of ADC Recovery and Other	0
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Reattribution	216
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	(18)	(35)	81
Prior Year Development Net of External Reinsurance After ADC Cessions	(14)
Reattribution of ADC Recovery and Other	(5)
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Reattribution	(19)
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	38	(47)	(57)
Prior Year Development Net of External Reinsurance After ADC Cessions	37
Reattribution of ADC Recovery and Other	0
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Reattribution	37
Other product lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	(469)	(299)	$ (162)
Prior Year Development Net of External Reinsurance After ADC Cessions	(448)
Reattribution of ADC Recovery and Other	(33)
Amortization of Deferred Gain at Inception	(7)
Prior Year Development After Amortization and Reattribution	(488)
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	(322)
Prior Year Development Net of External Reinsurance After ADC Cessions	(424)
Reattribution of ADC Recovery and Other	0
Amortization of Deferred Gain at Inception	(124)
Prior Year Development After Amortization and Reattribution	(548)
Businesses in run-off
Liability for Future Policy Benefit, by Product Segment [Line Items]
Prior Year Development Net of External Reinsurance Before ADC Cessions	106	196
Prior Year Development Net of External Reinsurance After ADC Cessions	(3)
Reattribution of ADC Recovery and Other	0
Amortization of Deferred Gain at Inception	0
Prior Year Development After Amortization and Reattribution	$ (3)
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