Reconciliation of Incurred Losses and ALAE (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	$ 2,739
Total liabilities for loss and ALAE, net of reinsurance	5,799
Reinsurance recoverable on unpaid losses	342	$ 238	$ 187	$ 298
Unallocated loss adjustment expense reserves	289
Total liabilities for gross loss and loss expense reserves	6,088	$ 5,646	$ 5,219	$ 5,035
Other Short-duration Insurance Product Line
Liability for Claims and Claims Adjustment Expense
Total liabilities for loss and ALAE, net of reinsurance	771
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	4,910
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	2,863
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	88
Total liabilities for loss and ALAE, net of reinsurance	2,135
Reinsurance recoverable on unpaid losses	30
Total liabilities for gross loss and loss expense reserves	2,165
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	2,061
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,433
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	295
Total liabilities for loss and ALAE, net of reinsurance	923
Reinsurance recoverable on unpaid losses	64
Total liabilities for gross loss and loss expense reserves	987
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	2,200
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,566
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	649
Reinsurance recoverable on unpaid losses	5
Total liabilities for gross loss and loss expense reserves	654
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	2,739
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	2,469
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	283
Reinsurance recoverable on unpaid losses	111
Total liabilities for gross loss and loss expense reserves	394
Personal Insurance | Personal Auto
Liability for Claims and Claims Adjustment Expense
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	1,933
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,659
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	280
Reinsurance recoverable on unpaid losses	33
Total liabilities for gross loss and loss expense reserves	313
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	1,779
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,621
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	160
Reinsurance recoverable on unpaid losses	21
Total liabilities for gross loss and loss expense reserves	181
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	658
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	333
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	326
Reinsurance recoverable on unpaid losses	8
Total liabilities for gross loss and loss expense reserves	$ 334