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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
Sep. 30, 2015
Dec. 31, 2014
Offsetting Liabilities [Line Items]    
Derivative Assets $ 40.3 $ 64.4
Derivative Liabilities (3.3) (8.0)
Net amounts recognized in Other assets 37.0 56.4
Over The Counter | Fixed income/interest rate    
Offsetting Liabilities [Line Items]    
Derivative Assets 0.5 1.0
Derivative Liabilities (0.8) (5.4)
Net amounts recognized in Other assets (0.3) (4.4)
Over The Counter | Foreign exchange    
Offsetting Liabilities [Line Items]    
Derivative Assets 27.9 45.5
Derivative Liabilities 0.0 0.0
Net amounts recognized in Other assets 27.9 45.5
Over The Counter | Equity    
Offsetting Liabilities [Line Items]    
Derivative Assets 11.5 11.7
Derivative Liabilities 0.0 (0.2)
Net amounts recognized in Other assets 11.5 11.5
Exchange Traded | Fixed income/interest rate    
Offsetting Liabilities [Line Items]    
Derivative Assets 0.3 2.8
Derivative Liabilities (0.1) (0.1)
Net amounts recognized in Other assets 0.2 2.7
Exchange Traded | Foreign exchange    
Offsetting Liabilities [Line Items]    
Derivative Assets 0.1 0.0
Derivative Liabilities (0.1) (1.4)
Net amounts recognized in Other assets 0.0 (1.4)
Exchange Traded | Equity    
Offsetting Liabilities [Line Items]    
Derivative Assets 0.0 3.4
Derivative Liabilities (2.3) (0.9)
Net amounts recognized in Other assets $ (2.3) $ 2.5