Reconciliation of Incurred Losses and ALAE (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Liability for Claims and Claims Adjustment Expense
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	$ 3,312
Total liabilities for loss and ALAE, net of reinsurance	7,967
Reinsurance recoverable on unpaid losses	405	$ 327	$ 277	$ 342
Unallocated loss adjustment expense reserves	369
Total liabilities for gross loss and loss expense reserves	8,336	$ 7,229	$ 6,677	$ 6,088
Other Short-duration Insurance Product Line
Liability for Claims and Claims Adjustment Expense
Total liabilities for loss and ALAE, net of reinsurance	1,512
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	5,932
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	3,396
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	86
Total liabilities for loss and ALAE, net of reinsurance	2,622
Reinsurance recoverable on unpaid losses	43
Total liabilities for gross loss and loss expense reserves	2,665
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	1,865
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,215
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	311
Total liabilities for loss and ALAE, net of reinsurance	961
Reinsurance recoverable on unpaid losses	57
Total liabilities for gross loss and loss expense reserves	1,018
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	2,332
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,581
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	40
Total liabilities for loss and ALAE, net of reinsurance	791
Reinsurance recoverable on unpaid losses	5
Total liabilities for gross loss and loss expense reserves	796
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	3,312
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	2,774
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	15
Total liabilities for loss and ALAE, net of reinsurance	553
Reinsurance recoverable on unpaid losses	47
Total liabilities for gross loss and loss expense reserves	600
Personal Insurance | Personal Auto
Liability for Claims and Claims Adjustment Expense
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	1,831
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,557
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	13
Total liabilities for loss and ALAE, net of reinsurance	287
Reinsurance recoverable on unpaid losses	28
Total liabilities for gross loss and loss expense reserves	315
Personal Insurance | Home Owner Line
Liability for Claims and Claims Adjustment Expense
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	2,275
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,952
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	329
Reinsurance recoverable on unpaid losses	8
Total liabilities for gross loss and loss expense reserves	337
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	1,272
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	587
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	2
Total liabilities for loss and ALAE, net of reinsurance	687
Reinsurance recoverable on unpaid losses	37
Total liabilities for gross loss and loss expense reserves	$ 724