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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

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