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Derivatives (Realized and Unrealized Gains (Losses)) (Details) - White Mountains Life Re - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |
Change in fair value of derivatives, included in other revenue | $ (8.8) | $ (72.4) | $ (402.0) |
Derivative, Fair Value, Net | 20.1 | 56.4 | 69.2 | $ 98.3
Fixed income/interest rate |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |
Change in fair value of derivatives, included in other revenue | 6.4 | (33.7) | (108.7) |
Derivative, Fair Value, Net | 0.5 | (1.7) |  |
Foreign exchange |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |
Change in fair value of derivatives, included in other revenue | (7.3) | (1.3) | (96.7) |
Derivative, Fair Value, Net | 14.8 | 44.1 |  |
Equity |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |
Change in fair value of derivatives, included in other revenue | (7.9) | (37.4) | $ (196.6) |
Derivative, Fair Value, Net | $ 4.8 | 14.0 |  |
Level 3 | Projected Surrender Assumption [Member] |  |  |  |
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments |  |  |  |
Change in fair value of derivatives, included in other revenue |  | $ 0.2 |  |

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