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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

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