|**Hardy (Tables)**                                                | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                       
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|**Hardy [Abstract]**                                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Schedule of fair value of assets acquired and liabilities assumed | (In millions)Acquisition DateJuly 2, 2012Investments:Fixed maturity securities$117Short term investments255Total investments372Cash34Reinsurance receivables252Insurance receivables222Accrued investment income2Property and equipment4Goodwill and other intangible assets171Other assets109Total assets acquired$1,166Claim and claim adjustment expenses$500Unearned premiums249Long term debt30Other liabilities156Total liabilities assumed$935
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|(In millions)                                                     | Acquisition Date                                                                                                                                                                                                                                                                                                                                                                                                                                     
|July 2, 2012                                                     
|Investments:                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Fixed maturity securities                                         | $                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Short term investments                                            | 255                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Total investments                                                 | 372                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Cash                                                              | 34                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Reinsurance receivables                                           | 252                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Insurance receivables                                             | 222                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Accrued investment income                                         | 2                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Property and equipment                                            | 4                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Goodwill and other intangible assets                              | 171                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Other assets                                                      | 109                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Total assets acquired                                             | $                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Claim and claim adjustment expenses                               | $                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Unearned premiums                                                 | 249                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Long term debt                                                    | 30                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Other liabilities                                                 | 156                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Total liabilities assumed                                         | $                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Schedule of intangible assets acquired                            | (In millions)AmountEconomic Useful LifeSyndicate capacity$55IndefiniteTotal indefinite-lived intangible assets55Value of business acquired601 - 4 yearsTrade name88 yearsDistribution channel1315 yearsTotal finite-lived intangible assets81Total intangible assets as of the acquisition date$136                                                                                                                                                  
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|(In millions)                                                     | Amount                                                                                                                                                                                                                                                                                                                                                                                                                                               
|Syndicate capacity                                                | $                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Total indefinite-lived intangible assets                          | 55                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                      
|Value of business acquired                                        | 60                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Trade name                                                        | 8                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Distribution channel                                              | 13                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Total finite-lived intangible assets                              | 81                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Total intangible assets as of the acquisition date                | $                                                                                                                                                                                                                                                                                                                                                                                                                                                    

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