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Derivatives Derivatives (Gross Asset (Gross liability) Amounts Offset Under Master Netting Agreements) (Details) (White Mountains Life Re, USD $)
In Millions, unless otherwise specified
Dec. 31, 2013
Dec. 31, 2012
Derivative [Line Items]    
Derivative Asset $ 106.0 [1] $ 206.5 [1],[2]
Derivative Liability (36.8) [1] (108.2) [1]
Net amounts recognized in Other Assets 69.2 [1] 98.3 [1]
Interest Rate Contract | Over the Counter [Member]
    
Derivative [Line Items]    
Derivative Asset 2.4 52.6 [2]
Derivative Liability (11.7) (26.9)
Net amounts recognized in Other Assets (9.3) 25.7
Interest Rate Contract | Exchange Traded [Member]
    
Derivative [Line Items]    
Derivative Asset 1.0 1.6 [2]
Derivative Liability (1.6) (0.2)
Net amounts recognized in Other Assets (0.6) 1.4
Foreign Exchange Contract | Over the Counter [Member]
    
Derivative [Line Items]    
Derivative Asset 67.8 87.8 [2]
Derivative Liability (12.0) (34.4)
Net amounts recognized in Other Assets 55.8 53.4
Foreign Exchange Contract | Exchange Traded [Member]
    
Derivative [Line Items]    
Derivative Asset 2.3 0.8 [2]
Derivative Liability 0 (1.4)
Net amounts recognized in Other Assets 2.3 (0.6)
Equity Contract | Over the Counter [Member]
    
Derivative [Line Items]    
Derivative Asset 30.7 63.6 [2]
Derivative Liability (9.2) (22.9)
Net amounts recognized in Other Assets 21.5 40.7
Equity Contract | Exchange Traded [Member]
    
Derivative [Line Items]    
Derivative Asset 1.8 0.1 [2]
Derivative Liability (2.3) (22.4)
Net amounts recognized in Other Assets $ (0.5) $ (22.3)
[1] All derivative instruments held by WM Life Re are subject to master netting arrangements.
[2] Amount equal to fair value of instrument as recognized in other assets.