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Investments in Unconsolidated Affiliates (Investments in Symetra) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2014 | Sep. 30, 2013 | Sep. 30, 2014 | Sep. 30, 2013 | Jun. 20, 2014
Changes in carrying value of investment in Symetra |  |  |  |  |
Carrying value at the beginning of the period |  |  | $ 321.4 |  |
Equity in earnings | 7.0 | 8.6 | 33.3 | 24.9 |
Carrying value at the end of the period | 413.5 |  | 413.5 |  |
Income Tax Expense (Benefit) | 7.8 | 8.2 | 62.0 | 49.2 |
Symetra |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |
Carrying value at the beginning of the period | 415.5 | 326.0 | 317.3 | 381.5 |
Equity in earnings | 7.0 | 8.7 | 34.5 | 25.7 |
Equity in net unrealized losses from Symetra's fixed maturity portfolio | (10.7) | (7.7) | 64.0 | (88.2) |
Dividends received | (2.0) | (1.8) | (6.0) | (4.6) |
Increase (decrease) in value of warrants | 0 | 0 | 0 | 10.8 |
Carrying value at the end of the period | 409.8 | 325.2 | 409.8 | 325.2 |
Exercise of warrants | 0 | 0 | 0 | 0 |
Symetra | Common shareholders' equity. |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |
Share Price | $ 22.70 |  | $ 22.70 |  |
Changes in carrying value of investment in Symetra |  |  |  |  |
Carrying value at the beginning of the period | 415.5 | 326.0 | 317.3 | 351.2 |
Equity in earnings | 7.0 | 8.7 | 34.5 | 25.7 |
Equity in net unrealized losses from Symetra's fixed maturity portfolio | (10.7) | (7.7) | 64.0 | (88.2) |
Dividends received | (2.0) | (1.8) | (6.0) | (4.6) |
Increase (decrease) in value of warrants | 0 | 0 | 0 | 0 |
Carrying value at the end of the period | 409.8 | 325.2 | 409.8 | 325.2 |
Aggregate value of White Mountains common shares of Symetra at quoted market price | 455.1 |  | 455.1 |  |
Exercise of warrants | 0 | 0 | 0 | 41.1 |
Symetra | Warrants |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |
Share Price |  |  |  |  | $ 15.53
Changes in carrying value of investment in Symetra |  |  |  |  |
Carrying value at the beginning of the period | 0 | 0 | 0 | 30.3 |
Equity in earnings | 0 | 0 | 0 | 0 |
Equity in net unrealized losses from Symetra's fixed maturity portfolio | 0 | 0 | 0 | 0 |
Dividends received | 0 | 0 | 0 | 0 |
Increase (decrease) in value of warrants | 0 | 0 | 0 | 10.8 |
Carrying value at the end of the period | 0 | 0 | 0 | 0 |
Exercise of warrants | 0 | 0 | 0 | (41.1) |
White Mountains | Common shareholders' equity. |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |
Equity in net unrealized losses from Symetra's fixed maturity portfolio | 20.5 | (25.5) | 20.5 | (25.5) |
equity in earnings of unconsolidated affiliates | Symetra | Common shareholders' equity. |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |
Income Tax Expense (Benefit) | 0.5 | 0.6 | 2.4 | 2.0 |
Net unrealized gains (losses) of unconsolidated affiliates | White Mountains | Common shareholders' equity. |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |
Income Tax Expense (Benefit) | (1.4) | 2.0 | (1.4) | 2.0 |
Amortization of common share basis difference | Symetra | Common shareholders' equity. |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |
Equity in earnings | 0.7 | 0.7 | 2.2 | 2.2 |
impairment of net unrealized gains (losses) | Net unrealized gains (losses) of unconsolidated affiliates | Symetra | Common shareholders' equity. |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |
Equity in net unrealized losses from Symetra's fixed maturity portfolio | $ 3.1 | $ 3.1 | $ 9.4 | $ 8.6 |

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