|**Investments (Net investment income) (Details) (USD $)In Millions, unless otherwise specified**                                                                                                                           | 3 Months Ended                                                                                                                                                                                           | 6 Months Ended                                                                                                                                                                                          
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|**Net Investment Income**                                                                                                                                                                                                  |                                                                                                                                                                                                          |                                                                                                                                                                                                         
|Gross investment income                                                                                                                                                                                                    | $ 485                                                                                                                                                                                                    |                                                                                                                                                                                                         
|Investment expense                                                                                                                                                                                                         | (15)                                                                                                                                                                                                     |                                                                                                                                                                                                         
|Net investment income                                                                                                                                                                                                      | 470                                                                                                                                                                                                      |                                                                                                                                                                                                         
|Fixed maturity securities [Member]                                                                                                                                                                                         |                                                                                                                                                                                                          |                                                                                                                                                                                                         
|**Net Investment Income**                                                                                                                                                                                                  |                                                                                                                                                                                                          |                                                                                                                                                                                                         
|Gross investment income                                                                                                                                                                                                    | 505                                                                                                                                                                                                      |                                                                                                                                                                                                         
|Short term investments [Member]                                                                                                                                                                                            |                                                                                                                                                                                                          |                                                                                                                                                                                                         
|**Net Investment Income**                                                                                                                                                                                                  |                                                                                                                                                                                                          |                                                                                                                                                                                                         
|Gross investment income                                                                                                                                                                                                    | 2                                                                                                                                                                                                        |                                                                                                                                                                                                         
|Limited partnership investments [Member]                                                                                                                                                                                   |                                                                                                                                                                                                          |                                                                                                                                                                                                         
|**Net Investment Income**                                                                                                                                                                                                  |                                                                                                                                                                                                          |                                                                                                                                                                                                         
|Gross investment income                                                                                                                                                                                                    | (35)                                                                                                                                                                                                     |                                                                                                                                                                                                         
|Equity securities [Member]                                                                                                                                                                                                 |                                                                                                                                                                                                          |                                                                                                                                                                                                         
|**Net Investment Income**                                                                                                                                                                                                  |                                                                                                                                                                                                          |                                                                                                                                                                                                         
|Gross investment income                                                                                                                                                                                                    | 2                                                                                                                                                                                                        |                                                                                                                                                                                                         
|Mortgage loans [Member]                                                                                                                                                                                                    |                                                                                                                                                                                                          |                                                                                                                                                                                                         
|**Net Investment Income**                                                                                                                                                                                                  |                                                                                                                                                                                                          |                                                                                                                                                                                                         
|Gross investment income                                                                                                                                                                                                    | 5                                                                                                                                                                                                        |                                                                                                                                                                                                         
|Trading portfolio [Member]                                                                                                                                                                                                 |                                                                                                                                                                                                          |                                                                                                                                                                                                         
|**Net Investment Income**                                                                                                                                                                                                  |                                                                                                                                                                                                          |                                                                                                                                                                                                         
|Gross investment income                                                                                                                                                                                                    | 4                                                                                                                                                                                                        | [1]                                                                                                                                                                                                     
|Other [Member]                                                                                                                                                                                                             |                                                                                                                                                                                                          |                                                                                                                                                                                                         
|**Net Investment Income**                                                                                                                                                                                                  |                                                                                                                                                                                                          |                                                                                                                                                                                                         
|Gross investment income                                                                                                                                                                                                    | $ 2                                                                                                                                                                                                      |                                                                                                                                                                                                         
|                                                                                                                                                                                                                          
|[1]
              There were no net unrealized gains (losses) related to changes in fair value of trading securities still held included in net investment income for the three or six months ended June 30, 2012 or 2011. | [1]                                                                                                                                                                                                      | There were no net unrealized gains (losses) related to changes in fair value of trading securities still held included in net investment income for the three or six months ended June 30, 2012 or 2011.
|[1]                                                                                                                                                                                                                        | There were no net unrealized gains (losses) related to changes in fair value of trading securities still held included in net investment income for the three or six months ended June 30, 2012 or 2011.

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