Property Casualty Loss And Loss Expenses (Reconciliation of Incurred Losses and ALAE) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	$ 2,353
Total liabilities for loss and ALAE, net of reinsurance	4,930
Reinsurance recoverable on unpaid losses	187	$ 298	$ 281	$ 282
Unallocated loss adjustment expense reserves	289
Total liabilities for gross loss and loss expense reserves	5,219	$ 5,035	$ 4,660	$ 4,438
Other Short-duration Insurance Product Line
Liability for Claims and Claims Adjustment Expense [Line Items]
Total liabilities for loss and ALAE, net of reinsurance	315
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	4,434
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	2,573
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	83
Total liabilities for loss and ALAE, net of reinsurance	1,944
Reinsurance recoverable on unpaid losses	43
Total liabilities for gross loss and loss expense reserves	1,987
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	2,259
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,598
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	267
Total liabilities for loss and ALAE, net of reinsurance	928
Reinsurance recoverable on unpaid losses	64
Total liabilities for gross loss and loss expense reserves	992
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	1,955
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,380
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	16
Total liabilities for loss and ALAE, net of reinsurance	591
Reinsurance recoverable on unpaid losses	4
Total liabilities for gross loss and loss expense reserves	595
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	2,353
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	2,095
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	12
Total liabilities for loss and ALAE, net of reinsurance	270
Reinsurance recoverable on unpaid losses	7
Total liabilities for gross loss and loss expense reserves	277
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	1,774
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,503
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	276
Reinsurance recoverable on unpaid losses	31
Total liabilities for gross loss and loss expense reserves	307
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	1,466
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,358
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	111
Reinsurance recoverable on unpaid losses	3
Total liabilities for gross loss and loss expense reserves	114
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	441
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	195
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	340
Reinsurance recoverable on unpaid losses	3
Total liabilities for gross loss and loss expense reserves	$ 343