|**Investments (Net investment income) (Details) (USD $)In Millions, unless otherwise specified**                                                                                                                                                                                                                                                                                                  | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
|
|**Net Investment Income**                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                                                                                                
|Gross investment income                                                                                                                                                                                                                                                                                                                                                                           | $ 2,112                                                                                                                                                                                                                                                                                                                                                                        
|Investment expense                                                                                                                                                                                                                                                                                                                                                                                | (58)                                                                                                                                                                                                                                                                                                                                                                           
|Net investment income                                                                                                                                                                                                                                                                                                                                                                             | 2,054                                                                                                                                                                                                                                                                                                                                                                          
|Fixed maturity securities [Member]                                                                                                                                                                                                                                                                                                                                                                |                                                                                                                                                                                                                                                                                                                                                                                
|**Net Investment Income**                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                                                                                                
|Gross investment income                                                                                                                                                                                                                                                                                                                                                                           | 2,011                                                                                                                                                                                                                                                                                                                                                                          
|Short term investments [Member]                                                                                                                                                                                                                                                                                                                                                                   |                                                                                                                                                                                                                                                                                                                                                                                
|**Net Investment Income**                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                                                                                                
|Gross investment income                                                                                                                                                                                                                                                                                                                                                                           | 8                                                                                                                                                                                                                                                                                                                                                                              
|Limited partnership investments [Member]                                                                                                                                                                                                                                                                                                                                                          |                                                                                                                                                                                                                                                                                                                                                                                
|**Net Investment Income**                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                                                                                                
|Gross investment income                                                                                                                                                                                                                                                                                                                                                                           | 48                                                                                                                                                                                                                                                                                                                                                                             
|Equity securities [Member]                                                                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                                                                                
|**Net Investment Income**                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                                                                                                
|Gross investment income                                                                                                                                                                                                                                                                                                                                                                           | 20                                                                                                                                                                                                                                                                                                                                                                             
|Mortgage loans [Member]                                                                                                                                                                                                                                                                                                                                                                           |                                                                                                                                                                                                                                                                                                                                                                                
|**Net Investment Income**                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                                                                                                
|Gross investment income                                                                                                                                                                                                                                                                                                                                                                           | 9                                                                                                                                                                                                                                                                                                                                                                              
|Trading portfolio [Member]                                                                                                                                                                                                                                                                                                                                                                        |                                                                                                                                                                                                                                                                                                                                                                                
|**Net Investment Income**                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                                                                                                
|Gross investment income                                                                                                                                                                                                                                                                                                                                                                           | 9                                                                                                                                                                                                                                                                                                                                                                              
|Other [Member]                                                                                                                                                                                                                                                                                                                                                                                    |                                                                                                                                                                                                                                                                                                                                                                                
|**Net Investment Income**                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                                                                                                
|Gross investment income                                                                                                                                                                                                                                                                                                                                                                           | $ 7                                                                                                                                                                                                                                                                                                                                                                            
|                                                                                                                                                                                                                                                                                                                                                                                                 
|[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 years ended December 31, 2011 and 2010. Net unrealized losses related to changes in fair value on trading securities still held included in net investment income were $5 million for the year ended December 31, 2009. | [1]                                                                                                                                                                                                                                                                                                                                                                            
|[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 years ended December 31, 2011 and 2010. Net unrealized losses related to changes in fair value on trading securities still held included in net investment income were $5 million for the year ended December 31, 2009.

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