Property Casualty Loss And Loss Expenses (Reconciliation of Incurred Losses and ALAE) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Liability for Claims and Claims Adjustment Expense [Line Items]
Cumulative incurred losses and ALAE as reported within the triangles, net of reinsurance	$ 2,187
Total liabilities for loss and ALAE, net of reinsurance	4,770
Reinsurance recoverable on unpaid losses	298	$ 281	$ 282	$ 299
Unallocated loss adjustment expense reserves	265
Total liabilities for gross loss and loss expense reserves	5,035	$ 4,660	$ 4,438	$ 4,241
Other Short-duration Insurance Product Line
Liability for Claims and Claims Adjustment Expense [Line Items]
Total liabilities for loss and ALAE, net of reinsurance	342
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	4,160
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	2,421
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	81
Total liabilities for loss and ALAE, net of reinsurance	1,820
Reinsurance recoverable on unpaid losses	137
Total liabilities for gross loss and loss expense reserves	1,957
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	2,370
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,686
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	242
Total liabilities for loss and ALAE, net of reinsurance	926
Reinsurance recoverable on unpaid losses	75
Total liabilities for gross loss and loss expense reserves	1,001
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	1,768
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,264
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	516
Reinsurance recoverable on unpaid losses	5
Total liabilities for gross loss and loss expense reserves	521
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	2,187
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,910
Liabilities for loss and ALAE for accident years not presented in the triangles, net of reinsurance	11
Total liabilities for loss and ALAE, net of reinsurance	288
Reinsurance recoverable on unpaid losses	18
Total liabilities for gross loss and loss expense reserves	306
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	1,633
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,388
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	249
Reinsurance recoverable on unpaid losses	31
Total liabilities for gross loss and loss expense reserves	280
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	1,452
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	1,322
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	132
Reinsurance recoverable on unpaid losses	2
Total liabilities for gross loss and loss expense reserves	134
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	366
Cumulative paid losses and ALAE as reported within the triangles, net of reinsurance	144
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	223
Reinsurance recoverable on unpaid losses	6
Total liabilities for gross loss and loss expense reserves	$ 229