Property Casualty Loss And Loss Expenses (Reconciliation of Incurred Losses and ALAE) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	$ 2,588
Total liabilities for loss and ALAE, net of reinsurance	5,366
Reinsurance recoverable on unpaid losses	238	$ 187	$ 298	$ 281
Unallocated loss adjustment expense reserves	280
Total liabilities for gross loss and loss expense reserves	5,646	$ 5,219	$ 5,035	$ 4,660
Other Short-duration Insurance Product Line
Liability for Claims and Claims Adjustment Expense [Line Items]
Total liabilities for loss and ALAE, net of reinsurance	483
Commercial Insurance | Commercial Casualty Line
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	4,671
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	2,658
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	94
Total liabilities for loss and ALAE, net of reinsurance	2,107
Reinsurance recoverable on unpaid losses	51
Total liabilities for gross loss and loss expense reserves	2,158
Commercial Insurance | Workers' Compensation Line
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	2,142
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,493
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	283
Total liabilities for loss and ALAE, net of reinsurance	932
Reinsurance recoverable on unpaid losses	59
Total liabilities for gross loss and loss expense reserves	991
Commercial Insurance | Commercial Auto Line
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	2,104
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,486
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	14
Total liabilities for loss and ALAE, net of reinsurance	632
Reinsurance recoverable on unpaid losses	5
Total liabilities for gross loss and loss expense reserves	637
Commercial Insurance | Commercial Property Line
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	2,588
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	2,287
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	314
Reinsurance recoverable on unpaid losses	0
Total liabilities for gross loss and loss expense reserves	314
Personal Insurance | Personal Auto
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	1,889
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,603
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	290
Reinsurance recoverable on unpaid losses	34
Total liabilities for gross loss and loss expense reserves	324
Personal Insurance | Home Owner Line
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	1,614
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,474
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	3
Total liabilities for loss and ALAE, net of reinsurance	143
Reinsurance recoverable on unpaid losses	32
Total liabilities for gross loss and loss expense reserves	175
Excess and Surplus Lines Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	532
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	256
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	277
Reinsurance recoverable on unpaid losses	7
Total liabilities for gross loss and loss expense reserves	$ 284