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Investments in Unconsolidated Affiliates (Details) (USD $) In Millions, unless otherwise specified | Mar. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2012 | Dec. 31, 2011
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Investments in unconsolidated affiliates |  |  |  |  |  |
Total investments in unconsolidated affiliates | $ 383.6 |  | $ 387.9 |  |  |
Symetra |  |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |  |
Equity in Unrealized Gains (Losses) from Investments in Unconsolidated Affiliates | 54.1 |  | 62.8 |  |  |
Total investments in unconsolidated affiliates | 377.5 | [1],[2] | 381.5 | 293.2 | [1],[2] | 273.6
Symetra | Common shareholders' equity. |  |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |  |
Symetra Common Shares excluding unrealized gains (losses) | 296.8 |  | 288.4 |  |  |
Total investments in unconsolidated affiliates | 350.9 | [1],[2] | 351.2 | 269.5 | [1],[2] | 261.0
Symetra | Warrants |  |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |  |
Total investments in unconsolidated affiliates | 26.6 | [1],[2] | 30.3 | 23.7 | [1],[2] | 12.6
Hamer, LLC |  |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |  |
Total investments in unconsolidated affiliates | 3.8 |  | 4.0 |  |  |
Bri-Mar Manufacturing, LLC |  |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |  |
Total investments in unconsolidated affiliates | 1.8 |  | 1.9 |  |  |
Pentelia Capital Management |  |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |  |
Total investments in unconsolidated affiliates | $ 0.5 |  | $ 0.5 |  |  |
[1] | 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.
[2] | 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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- Definition Total investments in (A) companies that are accounted for using the equity method and (B) warrants to acquire additional shares of an unconsolidated affiliate that are carried at fair value. + References No definition available. + Details
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