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Derivatives (Realized and Unrealized Gains (Losses)) (Details) - White Mountains Life Re - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
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 | $ (2.0) | $ (8.8) | $ (72.4) |
Derivative, Fair Value, Net | 0.0 | 20.1 | 56.4 | $ 69.2
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 | 1.8 | 6.4 | (33.7) |
Derivative, Fair Value, Net | 0.0 | 0.5 |  |
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 | (4.8) | (7.3) | (1.3) |
Derivative, Fair Value, Net | 0.0 | 14.8 |  |
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 | 1.0 | (7.9) | (37.4) |
Derivative, Fair Value, Net | $ 0.0 | $ 4.8 |  |
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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