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Derivatives (Value, Collateral (held) provided by WM Life Re and Net Exposure to Credit Losses on OTC Derivative Instruments) (Details) (White Mountains Life Re, USD $)
In Millions, unless otherwise specified
Sep. 30, 2013
Dec. 31, 2012
Offsetting Liabilities [Line Items]    
Derivative Assets $ 127.5 [1],[2] $ 206.5 [1],[2]
Derivative Liabilities (36.0) [2] (108.2) [2]
Net amounts recognized in Other assets 91.5 [2] 98.3 [2]
Over The Counter | Fixed income/interest rate
    
Offsetting Liabilities [Line Items]    
Derivative Assets 0.2 [1] 52.6 [1]
Derivative Liabilities (13.1) (26.9)
Net amounts recognized in Other assets (12.9) 25.7
Over The Counter | Foreign exchange
    
Offsetting Liabilities [Line Items]    
Derivative Assets 80.7 [1] 87.8 [1]
Derivative Liabilities (10.1) (34.4)
Net amounts recognized in Other assets 70.6 53.4
Over The Counter | Equity
    
Offsetting Liabilities [Line Items]    
Derivative Assets 44.6 [1] 63.6 [1]
Derivative Liabilities (6.9) (22.9)
Net amounts recognized in Other assets 37.7 40.7
Exchange Traded | Fixed income/interest rate
    
Offsetting Liabilities [Line Items]    
Derivative Assets 0.7 [1] 1.6 [1]
Derivative Liabilities (3.2) (0.2)
Net amounts recognized in Other assets (2.5) 1.4
Exchange Traded | Foreign exchange
    
Offsetting Liabilities [Line Items]    
Derivative Assets 0.7 [1] 0.8 [1]
Derivative Liabilities (0.3) (1.4)
Net amounts recognized in Other assets 0.4 (0.6)
Exchange Traded | Equity
    
Offsetting Liabilities [Line Items]    
Derivative Assets 0.6 [1] 0.1 [1]
Derivative Liabilities (2.4) (22.4)
Net amounts recognized in Other assets $ (1.8) $ (22.3)
[1] Amount equal to fair value of instrument as recognized in other assets.
[2] All derivative instruments held by WM Life Re are subject to master netting arrangements.