|**Reinsurance (Tables)**                               | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
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|**Reinsurance Disclosures [Abstract]**                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Property Casualty Insurance Assumed And Ceded Business | The table below summarizes our consolidated property casualty insurance net written premiums, earned premiums and incurred loss and loss expenses:(Dollars in millions)Three months ended March 31,20202019Direct written premiums$1,458$1,335Assumed written premiums11187Ceded written premiums(51)(41)Net written premiums$1,518$1,381Direct earned premiums$1,371$1,266Assumed earned premiums6643Ceded earned premiums(48)(42)Earned premiums$1,389$1,267Direct incurred loss and loss expenses$900$787Assumed incurred loss and loss expenses3425Ceded incurred loss and loss expenses(4)(22)Incurred loss and loss expenses$930$790
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|(Dollars in millions)                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Direct written premiums                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Assumed written premiums                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Ceded written premiums                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Net written premiums                                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Direct earned premiums                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Assumed earned premiums                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Ceded earned premiums                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Earned premiums                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Direct incurred loss and loss expenses                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Assumed incurred loss and loss expenses                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Ceded incurred loss and loss expenses                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Incurred loss and loss expenses                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
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|Life Insurance Assumed and Ceded Business              | The table below summarizes our consolidated life insurance earned premiums and contract holders' benefits incurred:(Dollars in millions)Three months ended March 31,20202019Direct earned premiums$85$83Ceded earned premiums(18)(17)Earned premiums$67$66Direct contract holders' benefits incurred8585Ceded contract holders' benefits incurred(12)(15)Contract holders' benefits incurred$73$70                                                                                                                                                                                                                                        
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|(Dollars in millions)                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Direct earned premiums                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Ceded earned premiums                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Earned premiums                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
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|Direct contract holders' benefits incurred             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Ceded contract holders' benefits incurred              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|Contract holders' benefits incurred                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
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