Insurance Claim Reserves (Details) - Claims and Claim Adjustment Expenses Reserves Summary - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	$ 54,852
Discount (Net of Reinsurance)	(1,048)
Subtotal: Net Claims and Claim Adjustment Expense Reserves	53,804
Reinsurance Recoverables on Unpaid Losses	7,823
Claims and Claim Adjustment Expense Reserves	61,627	$ 58,649
Discount on reinsurance recoverables for long-term disability and annuity claim payments included in discount (net of reinsurance)	48
Reinsurance recoverables	8,143	8,063	$ 8,452
Property-casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	54,852
Discount (Net of Reinsurance)	(1,048)
Subtotal: Net Claims and Claim Adjustment Expense Reserves	53,804	50,853	48,688	$ 46,357
Reinsurance Recoverables on Unpaid Losses	7,817	7,790	8,209	8,153
Claims and Claim Adjustment Expense Reserves	61,621	58,643	$ 56,897	$ 54,510
Accident and health [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	0
Discount (Net of Reinsurance)	0
Subtotal: Net Claims and Claim Adjustment Expense Reserves	0
Reinsurance Recoverables on Unpaid Losses	6
Claims and Claim Adjustment Expense Reserves	6	$ 6
Business Insurance [Member] | General liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	10,251
Business Insurance [Member] | Commercial property [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	1,160
Business Insurance [Member] | Commercial multi-peril [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	5,205
Business Insurance [Member] | Automobile [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	4,263
Business Insurance [Member] | Workers' compensation [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	16,104
Bond & Specialty Insurance [Member] | General liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	2,432
Bond & Specialty Insurance [Member] | Fidelity and surety [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	611
Personal Insurance [Member] | Automobile [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	4,095
Personal Insurance [Member] | Homeowners (excluding Other) [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	2,439
Personal Insurance [Member] | International - Canada [Member] | Property-casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	765
Reportable segments [Member] | Property-casualty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	47,325
Discount (Net of Reinsurance)	(1,044)
Subtotal: Net Claims and Claim Adjustment Expense Reserves	46,281
Reinsurance Recoverables on Unpaid Losses	3,363
Claims and Claim Adjustment Expense Reserves	49,644
Reportable segments [Member] | Business Insurance [Member] | General liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	10,251
Discount (Net of Reinsurance)	(132)
Subtotal: Net Claims and Claim Adjustment Expense Reserves	10,119
Reinsurance Recoverables on Unpaid Losses	1,052
Claims and Claim Adjustment Expense Reserves	11,171
Reportable segments [Member] | Business Insurance [Member] | Commercial property [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	1,160
Discount (Net of Reinsurance)	0
Subtotal: Net Claims and Claim Adjustment Expense Reserves	1,160
Reinsurance Recoverables on Unpaid Losses	464
Claims and Claim Adjustment Expense Reserves	1,624
Reportable segments [Member] | Business Insurance [Member] | Commercial multi-peril [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	5,205
Discount (Net of Reinsurance)	0
Subtotal: Net Claims and Claim Adjustment Expense Reserves	5,205
Reinsurance Recoverables on Unpaid Losses	207
Claims and Claim Adjustment Expense Reserves	5,412
Reportable segments [Member] | Business Insurance [Member] | Automobile [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	4,263
Discount (Net of Reinsurance)	0
Subtotal: Net Claims and Claim Adjustment Expense Reserves	4,263
Reinsurance Recoverables on Unpaid Losses	299
Claims and Claim Adjustment Expense Reserves	4,562
Reportable segments [Member] | Business Insurance [Member] | Workers' compensation [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	16,104
Discount (Net of Reinsurance)	(912)
Subtotal: Net Claims and Claim Adjustment Expense Reserves	15,192
Reinsurance Recoverables on Unpaid Losses	614
Claims and Claim Adjustment Expense Reserves	15,806
Reportable segments [Member] | Bond & Specialty Insurance [Member] | General liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	2,432
Discount (Net of Reinsurance)	0
Subtotal: Net Claims and Claim Adjustment Expense Reserves	2,432
Reinsurance Recoverables on Unpaid Losses	276
Claims and Claim Adjustment Expense Reserves	2,708
Reportable segments [Member] | Bond & Specialty Insurance [Member] | Fidelity and surety [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	611
Discount (Net of Reinsurance)	0
Subtotal: Net Claims and Claim Adjustment Expense Reserves	611
Reinsurance Recoverables on Unpaid Losses	24
Claims and Claim Adjustment Expense Reserves	635
Reportable segments [Member] | Personal Insurance [Member] | Automobile [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	4,095
Discount (Net of Reinsurance)	0
Subtotal: Net Claims and Claim Adjustment Expense Reserves	4,095
Reinsurance Recoverables on Unpaid Losses	345
Claims and Claim Adjustment Expense Reserves	4,440
Reportable segments [Member] | Personal Insurance [Member] | Homeowners (excluding Other) [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	2,439
Discount (Net of Reinsurance)	0
Subtotal: Net Claims and Claim Adjustment Expense Reserves	2,439
Reinsurance Recoverables on Unpaid Losses	64
Claims and Claim Adjustment Expense Reserves	2,503
Reportable segments [Member] | Personal Insurance [Member] | International - Canada [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	765
Discount (Net of Reinsurance)	0
Subtotal: Net Claims and Claim Adjustment Expense Reserves	765
Reinsurance Recoverables on Unpaid Losses	18
Claims and Claim Adjustment Expense Reserves	783
Reconciling Items [Member] | Other insurance contracts [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	4,753
Discount (Net of Reinsurance)	(4)
Subtotal: Net Claims and Claim Adjustment Expense Reserves	4,749
Reinsurance Recoverables on Unpaid Losses	1,814
Claims and Claim Adjustment Expense Reserves	6,563
Reconciling Items [Member] | Unallocated loss adjustment expense reserves [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	2,689
Discount (Net of Reinsurance)	0
Subtotal: Net Claims and Claim Adjustment Expense Reserves	2,689
Reinsurance Recoverables on Unpaid Losses	14
Claims and Claim Adjustment Expense Reserves	2,703
Reconciling Items [Member] | Structured settlements [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	0
Discount (Net of Reinsurance)	0
Subtotal: Net Claims and Claim Adjustment Expense Reserves	0
Reinsurance Recoverables on Unpaid Losses	2,663
Claims and Claim Adjustment Expense Reserves	2,663
Reconciling Items [Member] | Other [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability
Net Undiscounted Claims and Claim Adjustment Expense Reserves	85
Discount (Net of Reinsurance)	0
Subtotal: Net Claims and Claim Adjustment Expense Reserves	85
Reinsurance Recoverables on Unpaid Losses	(37)
Claims and Claim Adjustment Expense Reserves	$ 48