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Derivatives Derivatives (Gross Asset (Gross liability) Amounts Offset Under Master Netting Agreements) (Details) - White Mountains Life Re - USD ($)
$ in Millions
Dec. 31, 2015
Dec. 31, 2014
Derivative [Line Items]    
Derivative Asset $ 23.2 $ 64.4
Derivative Liability (3.1) (8.0)
Net amounts recognized in Other Assets 20.1 56.4
Interest Rate Contract | Over the Counter [Member]    
Derivative [Line Items]    
Derivative Asset 2.4 1.0
Derivative Liability (2.1) (5.4)
Net amounts recognized in Other Assets 0.3 (4.4)
Interest Rate Contract | Exchange Traded [Member]    
Derivative [Line Items]    
Derivative Asset 0.1 2.8
Derivative Liability (0.1) (0.1)
Net amounts recognized in Other Assets 0.0 2.7
Foreign Exchange Contract | Over the Counter [Member]    
Derivative [Line Items]    
Derivative Asset 15.0 45.5
Derivative Liability 0.0 0.0
Net amounts recognized in Other Assets 15.0 45.5
Foreign Exchange Contract | Exchange Traded [Member]    
Derivative [Line Items]    
Derivative Asset 0.1 0.0
Derivative Liability (0.3) (1.4)
Net amounts recognized in Other Assets (0.2) (1.4)
Equity Contract | Over the Counter [Member]    
Derivative [Line Items]    
Derivative Asset 4.4 11.7
Derivative Liability (0.6) (0.2)
Net amounts recognized in Other Assets 3.8 11.5
Equity Contract | Exchange Traded [Member]    
Derivative [Line Items]    
Derivative Asset 1.2 3.4
Derivative Liability 0.0 (0.9)
Net amounts recognized in Other Assets $ 1.2 $ 2.5