Property Casualty Loss and Loss Expense - Reconciliation of Incurred Losses and ALAE (Details) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Liability for Claims and Claims Adjustment Expense
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	$ 3,496
Total liabilities for loss and ALAE, net of reinsurance	8,535
Reinsurance recoverable on unpaid losses	362	$ 405	$ 327	$ 277
Unallocated loss adjustment expense reserves	440
Total liabilities for gross loss and loss expense reserves	8,975	$ 8,336	$ 7,229	$ 6,677
Other Short-duration Insurance Product Line
Liability for Claims and Claims Adjustment Expense
Total liabilities for loss and ALAE, net of reinsurance	1,686
Commercial Insurance | Commercial Casualty Line
Liability for Claims and Claims Adjustment Expense
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	6,497
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	3,680
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	91
Total liabilities for loss and ALAE, net of reinsurance	2,908
Reinsurance recoverable on unpaid losses	19
Total liabilities for gross loss and loss expense reserves	2,927
Commercial Insurance | Workers' Compensation Line
Liability for Claims and Claims Adjustment Expense
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	1,826
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,160
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	316
Total liabilities for loss and ALAE, net of reinsurance	982
Reinsurance recoverable on unpaid losses	54
Total liabilities for gross loss and loss expense reserves	1,036
Commercial Insurance | Commercial Auto Line
Liability for Claims and Claims Adjustment Expense
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	2,432
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,657
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	32
Total liabilities for loss and ALAE, net of reinsurance	807
Reinsurance recoverable on unpaid losses	4
Total liabilities for gross loss and loss expense reserves	811
Commercial Insurance | Commercial Property Line
Liability for Claims and Claims Adjustment Expense
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	3,496
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	3,033
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	12
Total liabilities for loss and ALAE, net of reinsurance	475
Reinsurance recoverable on unpaid losses	50
Total liabilities for gross loss and loss expense reserves	525
Personal Insurance | Personal Auto
Liability for Claims and Claims Adjustment Expense
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	1,884
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,594
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	12
Total liabilities for loss and ALAE, net of reinsurance	302
Reinsurance recoverable on unpaid losses	25
Total liabilities for gross loss and loss expense reserves	327
Personal Insurance | Homeowner
Liability for Claims and Claims Adjustment Expense
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	2,576
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	2,231
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	6
Total liabilities for loss and ALAE, net of reinsurance	351
Reinsurance recoverable on unpaid losses	(3)
Total liabilities for gross loss and loss expense reserves	348
Excess and Surplus Lines Insurance
Liability for Claims and Claims Adjustment Expense
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	1,545
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	707
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	4
Total liabilities for loss and ALAE, net of reinsurance	842
Reinsurance recoverable on unpaid losses	33
Total liabilities for gross loss and loss expense reserves	$ 875