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Investments in Unconsolidated Affiliates (Details 2) (Symetra, USD $)
In Millions, except Share data, unless otherwise specified
 3 Months Ended 6 Months Ended 1 Months Ended
Mar. 31, 2010
Jun. 30, 2011
Jun. 30, 2011
Common shareholders' equity
Dec. 31, 2010
Common shareholders' equity
Jun. 30, 2011
Warrants
Jan. 31, 2010
Initial public offering
Investments in unconsolidated affiliates            
Investments in unconsolidated affiliates (in shares)     17,400,000 17,400,000    
Additional common shares (in shares)     9,500,000 9,500,000    
Risk free interest rate (as a percent)         0.84%  
Volatility rate (as a percent)         25.00%  
Expected life (in years)         3.08  
Strike price (in dollars per share)         $ 11.49  
Share price (in dollars per share)         $ 13.43  
Initial public offering issue price (in dollars per share)           $ 12.00
Sale of Symetra common shares by Symetra (in shares)           25,300,000
Sale of Symetra common shares by existing shareholders (in shares)           9,700,000
Ownership interest after initial public offering (as a percent) 20.00%          
Decrease in carrying value of Symetra from dilution $ 16          
Aggregate value of White Mountains common shares of Symetra at quoted market price   $ 233.7        
Quoted market price (in dollars per share)   $ 13.43