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Investments in Unconsolidated Affiliates (Investments in Symetra) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2013 | Mar. 31, 2012 | Dec. 31, 2011
Changes in carrying value of investment in Symetra |  |  |  |  |
Carrying value at the beginning of the period | $ 387.9 |  |  |  |
Equity in earnings | 9.2 |  | 10.2 |  |
Carrying value at the end of the period | 383.6 |  |  |  |
Symetra |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |
Carrying value at the beginning of the period | 381.5 |  | 273.6 |  |
Equity in earnings | 9.8 | [1],[2] | 10.9 | [1],[2] |
Equity in net unrealized losses from Symetra's fixed maturity portfolio | (8.7) | [3] | (1.2) | [3] |
Dividends received | (1.4) |  | (1.2) |  |
Increase (decrease) in value of warrants | 3.7 |  | (11.1) |  |
Carrying value at the end of the period | 377.5 | [4],[5] | 293.2 | [4],[5] | 273.6
Tax on equity in earnings | 0.8 |  | 0.7 |  |
Equity in net unrealized losses increase relating to amortization of common share impairment |  |  |  |  | 136.6
Tax on equity in net unrealized gains (losses) | 0.7 |  | 0 |  |
Aggregate value of White Mountains common shares of Symetra at quoted market price | 233.3 |  |  |  |
Symetra | Common shareholders' equity. |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |
Carrying value at the beginning of the period | 351.2 |  | 261.0 |  |
Equity in earnings | 9.8 | [1],[2] | 10.9 | [1],[2] |
Equity in net unrealized losses from Symetra's fixed maturity portfolio | (8.7) | [3] | (1.2) | [3] |
Dividends received | (1.4) |  | (1.2) |  |
Increase (decrease) in value of warrants | 0 |  | 0 |  |
Carrying value at the end of the period | 350.9 | [4],[5] | 269.5 | [4],[5] |
Impairment of Shares Change in Equity in Earnings | 0.7 |  | 1.1 |  |
Equity in net unrealized losses increase relating to amortization of common share impairment | 2.7 |  | 3.2 |  |
Symetra | Warrants |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |
Carrying value at the beginning of the period | 30.3 |  | 12.6 |  |
Equity in earnings | 0 | [1],[2] | 0 | [1],[2] |
Equity in net unrealized losses from Symetra's fixed maturity portfolio | 0 | [3] | 0 | [3] |
Dividends received | 0 |  | 0 |  |
Increase (decrease) in value of warrants | 3.7 |  | (11.1) |  |
Carrying value at the end of the period | 26.6 | [4],[5] | 23.7 | [4],[5] |
Cash dividends received on warrants | $ 0.8 |  |  |  |
[1] | Equity in earnings includes $0.7 and $1.1 increase relating to the pre-tax amortization of Symetra common share impairment from March 31, 2013 and 2012.
[2] | Equity in earnings excludes tax expense of $0.8 and $0.7.
[3] | Net unrealized gains includes $2.7 and $3.2 increase relating to the pre-tax amortization of Symetra common share impairment from March 31, 2013 and 2012.
[4] | Includes White Mountains’ equity in net unrealized gains from Symetra’s fixed maturity portfolio of $(8.7) and $(1.2) as of March 31, 2013 and 2012, which exclude tax benefit of $0.7 and $0.0.
[5] | The aggregate value of White Mountains’ investment in common shares of Symetra was $233.3 based upon the quoted market price of $13.41 per share at March 31, 2013.

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