Insurance Liabilities - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022	Jan. 20, 2017
Liability for Future Policy Benefit, by Product Segment [Line Items]
Contractual deductible recoverable amount	$ 12,100	$ 12,100
Collateral held for deductible recoverable amounts	8,600	8,700
Liability for unpaid losses and loss adjustment expenses, allowance for credit losses	14	14
Unfavorable (favorable) prior year loss reserve development	254	(392)	$ (530)
Unfavorable (favorable) prior year development, net of external reinsurance, before ADC cessions	254
Net loss reserve discount	$ 1,175	1,233
Tabular discount rate (as a percent)	45.00%
Workers compensation tabular discount amount	$ 107	294
Workers compensation non tabular discount amount	1,100	939
Net loss reserve discount charge	$ 226	195	(703)
New York
Liability for Future Policy Benefit, by Product Segment [Line Items]
Nontabular discount rate (as a percent)	5.00%
DELAWARE
Liability for Future Policy Benefit, by Product Segment [Line Items]
Nontabular discount rate (as a percent)	4.50%
DELAWARE | Lexington Insurance Company
Liability for Future Policy Benefit, by Product Segment [Line Items]
Nontabular discount rate (as a percent)	4.50%
PENNSYLVANIA | National Union Fire Insurance Company
Liability for Future Policy Benefit, by Product Segment [Line Items]
Nontabular discount rate (as a percent)	4.50%
NICO
Liability for Future Policy Benefit, by Product Segment [Line Items]
Amortization of deferred gain on retroactive reinsurance	$ 322	48	189
U.S. Workers' Compensation
Liability for Future Policy Benefit, by Product Segment [Line Items]
Unfavorable (favorable) prior year development, net of external reinsurance, before ADC cessions	(273)	(267)	(644)
U.S. Excess Casualty
Liability for Future Policy Benefit, by Product Segment [Line Items]
Unfavorable (favorable) prior year development, net of external reinsurance, before ADC cessions	545	(32)	(116)
U.S. Other Casualty
Liability for Future Policy Benefit, by Product Segment [Line Items]
Unfavorable (favorable) prior year development, net of external reinsurance, before ADC cessions	12	(133)	(149)
U.S. Financial Lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Unfavorable (favorable) prior year development, net of external reinsurance, before ADC cessions	51	94	939
U.S. Property and Special Risks
Liability for Future Policy Benefit, by Product Segment [Line Items]
Unfavorable (favorable) prior year development, net of external reinsurance, before ADC cessions	(44)	(10)	(81)
U.S. Personal Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Unfavorable (favorable) prior year development, net of external reinsurance, before ADC cessions	(23)	(64)
UK/Europe Casualty and Financial lines
Liability for Future Policy Benefit, by Product Segment [Line Items]
Unfavorable (favorable) prior year development, net of external reinsurance, before ADC cessions	170	165	82
UK/Europe Property and Special Risks
Liability for Future Policy Benefit, by Product Segment [Line Items]
Unfavorable (favorable) prior year development, net of external reinsurance, before ADC cessions	(35)	81	(153)
UK/Europe and Japan Personal Insurance
Liability for Future Policy Benefit, by Product Segment [Line Items]
Unfavorable (favorable) prior year development, net of external reinsurance, before ADC cessions	(47)	$ (57)	$ (111)
U.S. Run-Off Long Tail Insurance Lines | NICO | Accident Years 2015 and Prior
Liability for Future Policy Benefit, by Product Segment [Line Items]
Risk Transferred - U.S. Commercial long-tail exposures for accident years 2015 and prior (as a percent)				80.00%
Businesses in run-off
Liability for Future Policy Benefit, by Product Segment [Line Items]
Unfavorable (favorable) prior year development, net of external reinsurance, before ADC cessions	196
Businesses in run-off | Asbestos Loss Portfolio Transfer
Liability for Future Policy Benefit, by Product Segment [Line Items]
Unfavorable (favorable) prior year development, net of external reinsurance, before ADC cessions	85
Businesses in run-off | Blackboard
Liability for Future Policy Benefit, by Product Segment [Line Items]
Unfavorable (favorable) prior year development, net of external reinsurance, before ADC cessions	$ 112