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Investments in Unconsolidated Affiliates (Details) (USD $) In Millions, unless otherwise specified | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2012 | Jun. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2011
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Investments in unconsolidated affiliates |  |  |  |  |  |  |  |
Total investments in unconsolidated affiliates | $ 331.9 |  |  | $ 387.9 |  |  |  |
Symetra |  |  |  |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |  |  |  |
Equity in Unrealized Gains (Losses) from Investments in Unconsolidated Affiliates | (17.8) |  |  | 62.8 |  |  |  |
Total investments in unconsolidated affiliates | 326.0 | [1],[2],[3] | 377.5 | 381.5 | 335.7 | [1],[2],[3] | 293.2 | 273.6
Symetra | Common shareholders' equity. |  |  |  |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |  |  |  |
Symetra Common Shares excluding unrealized gains (losses) | 343.8 |  |  | 288.4 |  |  |  |
Total investments in unconsolidated affiliates | 326.0 | [1],[2],[3] | 350.9 | 351.2 | 305.9 | [1],[2],[3] | 269.5 | 261.0
Symetra | Warrants |  |  |  |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |  |  |  |
Total investments in unconsolidated affiliates | 0 | [1],[2],[3] | 26.6 | 30.3 | 29.8 | [1],[2],[3] | 23.7 | 12.6
Hamer, LLC |  |  |  |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |  |  |  |
Total investments in unconsolidated affiliates | 3.5 |  |  | 4.0 |  |  |  |
Bri-Mar Manufacturing, LLC |  |  |  |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |  |  |  |
Total investments in unconsolidated affiliates | 2.4 |  |  | 1.9 |  |  |  |
Pentelia Capital Management |  |  |  |  |  |  |  |
Investments in unconsolidated affiliates |  |  |  |  |  |  |  |
Total investments in unconsolidated affiliates | $ 0 |  |  | $ 0.5 |  |  |  |
[1] | The aggregate value of White Mountains’ investment in common shares of Symetra was $320.6 based upon the quoted market price of $15.99 per share at June 30, 2013.
[2] | Includes White Mountains’ equity in net unrealized (losses) gains from Symetra’s fixed maturity portfolio of $(17.8) and $29.4 as of June 30, 2013 and 2012, which exclude deferred tax assets (liabilities) of $1.4 and $(2.4).
[3] | Includes White Mountains’ equity in net unrealized gains from Symetra’s fixed maturity portfolio of $(17.8) and $29.4 as of June 30, 2013 and 2012, which exclude deferred tax assets (liabilities) of $1.4 and $(2.4).

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