Reconciliation of Incurred Losses and ALAE (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Claims and Claims Adjustment Expense
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	$ 3,158
Total liabilities for loss and ALAE, net of reinsurance	6,394
Reinsurance recoverable on unpaid losses	277	$ 342	$ 238	$ 187
Unallocated loss adjustment expense reserves	283
Total liabilities for gross loss and loss expense reserves	6,677	$ 6,088	$ 5,646	$ 5,219
Other Short-duration Insurance Product Line
Liability for Claims and Claims Adjustment Expense
Total liabilities for loss and ALAE, net of reinsurance	959
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	5,179
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	3,012
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	2,258
Reinsurance recoverable on unpaid losses	24
Total liabilities for gross loss and loss expense reserves	2,282
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	1,990
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,357
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	308
Total liabilities for loss and ALAE, net of reinsurance	941
Reinsurance recoverable on unpaid losses	61
Total liabilities for gross loss and loss expense reserves	1,002
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	2,235
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	26
Total liabilities for loss and ALAE, net of reinsurance	680
Reinsurance recoverable on unpaid losses	5
Total liabilities for gross loss and loss expense reserves	685
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	3,158
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	2,725
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	448
Reinsurance recoverable on unpaid losses	66
Total liabilities for gross loss and loss expense reserves	514
Personal Insurance | Personal Auto
Liability for Claims and Claims Adjustment Expense
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	1,860
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,610
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	1,996
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,781
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	219
Reinsurance recoverable on unpaid losses	18
Total liabilities for gross loss and loss expense reserves	237
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	836
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	430
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	407
Reinsurance recoverable on unpaid losses	16
Total liabilities for gross loss and loss expense reserves	$ 423