|**Property Casualty Loss and Loss Expenses (Tables)**         | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              
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|**Premiums Written, Net [Abstract]**                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Consolidated Property Casualty Loss And Loss Expense Reserves | This table summarizes activity for our consolidated property casualty loss and loss expense reserves:(Dollars in millions)Three months ended March 31,20162015Gross loss and loss expense reserves, beginning of period$4,660$4,438Less reinsurance recoverable281282Net loss and loss expense reserves, beginning of period4,3794,156Net incurred loss and loss expenses related to:Current accident year723711Prior accident years(62)(22)Total incurred661689Net paid loss and loss expenses related to:Current accident year146147Prior accident years416399Total paid562546Net loss and loss expense reserves, end of period4,4784,299Plus reinsurance recoverable272278Gross loss and loss expense reserves, end of period$4,750$4,577
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|(Dollars in millions)                                         | Three months ended March 31,                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|                                                              | 2016                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Gross loss and loss expense reserves, beginning of period     | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Less reinsurance recoverable                                  | 281                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Net loss and loss expense reserves, beginning of period       | 4,379                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Net incurred loss and loss expenses related to:               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Current accident year                                         | 723                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Prior accident years                                          | (62                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Total incurred                                                | 661                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Net paid loss and loss expenses related to:                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|Current accident year                                         | 146                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Prior accident years                                          | 416                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Total paid                                                    | 562                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Net loss and loss expense reserves, end of period             | 4,478                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Plus reinsurance recoverable                                  | 272                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
|Gross loss and loss expense reserves, end of period           | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
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duration X- DefinitionTabular disclosure of the activity in the reserve for settling insured claims and expenses incurred in the claims settlement process for the period. The estimated liability includes the amount of money that will be required for future payments of (a) claims that have been reported to the insurer, (b) claims related to insured events that have occurred but that have not been reported to the insurer as of the date the liability is estimated, and (c) claim adjustment expenses. Claim adjustment expenses include costs incurred in the claim settlement process such as legal fees; outside adjuster fees; and costs to record, process, and adjust claims.
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