Reconciliation of Incurred Losses and ALAE (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Liability for Claims and Claims Adjustment Expense
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	$ 3,118
Total liabilities for loss and ALAE, net of reinsurance	6,917
Reinsurance recoverable on unpaid losses	327	$ 277	$ 342	$ 238
Unallocated loss adjustment expense reserves	312
Total liabilities for gross loss and loss expense reserves	7,229	$ 6,677	$ 6,088	$ 5,646
Other Short-duration Insurance Product Line
Liability for Claims and Claims Adjustment Expense
Total liabilities for loss and ALAE, net of reinsurance	1,210
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	5,383
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	3,146
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	90
Total liabilities for loss and ALAE, net of reinsurance	2,327
Reinsurance recoverable on unpaid losses	33
Total liabilities for gross loss and loss expense reserves	2,360
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	1,923
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,279
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	301
Total liabilities for loss and ALAE, net of reinsurance	945
Reinsurance recoverable on unpaid losses	65
Total liabilities for gross loss and loss expense reserves	1,010
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	2,203
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,515
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	34
Total liabilities for loss and ALAE, net of reinsurance	722
Reinsurance recoverable on unpaid losses	4
Total liabilities for gross loss and loss expense reserves	726
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	3,118
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	2,698
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	17
Total liabilities for loss and ALAE, net of reinsurance	437
Reinsurance recoverable on unpaid losses	46
Total liabilities for gross loss and loss expense reserves	483
Personal Insurance | Personal Auto
Liability for Claims and Claims Adjustment Expense
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	1,802
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,552
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	10
Total liabilities for loss and ALAE, net of reinsurance	260
Reinsurance recoverable on unpaid losses	32
Total liabilities for gross loss and loss expense reserves	292
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	2,169
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,893
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	5
Total liabilities for loss and ALAE, net of reinsurance	281
Reinsurance recoverable on unpaid losses	8
Total liabilities for gross loss and loss expense reserves	289
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	1,021
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	496
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	1
Total liabilities for loss and ALAE, net of reinsurance	526
Reinsurance recoverable on unpaid losses	21
Total liabilities for gross loss and loss expense reserves	$ 547