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