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Derivatives (Realized and Unrealized Gains (Losses)) (Details) (White Mountains Life Re, USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments      
Change in fair value of derivatives, included in other revenues $ (339.0) $ 92.9 $ 127.0
Derivative, Fair Value, Net 98.3 290.4  
Fixed income/interest rate
      
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments      
Change in fair value of derivatives, included in other revenues (149.5) 8.9 17.6
Derivative, Fair Value, Net 4.3 31.1  
Foreign exchange
      
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments      
Change in fair value of derivatives, included in other revenues (102.3) 29.5 144.6
Derivative, Fair Value, Net 51.3 161.3  
Equity
      
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments      
Change in fair value of derivatives, included in other revenues (87.2) 54.5 (35.2)
Derivative, Fair Value, Net $ 42.7 $ 98.0