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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 | 12 Months Ended
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Dec. 31, 2012 | Sep. 30, 2012 | Jun. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2011 | Sep. 30, 2011 | Jun. 30, 2011 | Mar. 31, 2011 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value at the beginning of the period |  |  |  |  | $ 275.3 |  |  |  |  |  |  |  | $ 275.3 |  |  |  |  |
Equity in earnings (losses) of unconsolidated affiliates | 5.5 |  | 7.7 | 6.5 | 10.2 |  | (36.4) |  | 1.5 | 7.9 | 6.8 |  | 29.9 |  | (20.2) |  | 9.9 |
Impairment of equity in earnings of Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | [1] |  |
Impairment of net unrealized gains from Symetra's fixed maturity portolio |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | [2] |  |
Carrying value at the end of the period | 387.9 |  |  |  |  |  | 275.3 |  |  |  |  |  | 387.9 |  | 275.3 |  |  |
Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value at the beginning of the period |  |  |  |  | 273.6 | [3] |  |  |  |  | 387.5 | [3] | 273.6 | [3] | 387.5 | [3] | 307.7 | [3]
Equity in earnings (losses) of unconsolidated affiliates |  |  |  |  |  |  |  |  |  |  |  |  | 32.3 | [4],[5],[6] | 28.2 | [4],[5] | 11.1 | [4],[5],[7]
Impairment of equity in earnings of Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (50.0) | [1] |  |
Equity in net unrealized losses from Symetra's fixed maturity portfolio |  |  |  |  |  |  |  |  |  |  |  |  | 62.8 | [8] | 85.0 | [8] | 72.7 | [8],[9]
Impairment of net unrealized gains from Symetra's fixed maturity portolio |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (148.6) | [2] |  |
Dividends received |  |  |  |  |  |  |  |  |  |  |  |  | (4.9) |  | (4.0) |  | (2.6) |
Increase (decrease) in value of warrants |  |  |  |  |  |  |  |  |  |  |  |  | 17.7 |  | (24.5) |  | (1.4) |
Carrying value at the end of the period | 381.5 | [10],[3] |  |  |  |  | 273.6 | [3] |  |  |  |  | 381.5 | [10],[3] | 273.6 | [3] | 387.5 | [3]
Tax on equity in earnings |  |  |  |  |  |  |  |  |  |  |  |  | 2.6 |  | 2.3 |  | 1.4 |
Unrealized gains from Symetra’s fixed maturity portfolio |  |  |  |  |  |  |  |  |  |  |  |  | 62.8 |  | 0 |  | 63.7 |
Tax on equity in net unrealized gains (losses) |  |  |  |  |  |  |  |  |  |  |  |  | 5.1 |  | 0 |  | 5.2 |
Loss from silutive effect resulting from public offering |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 17.9 |
Tax Benefit on other than temporary impairment |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 4.1 |  |  |
Impairment of net unrealized gains |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 12.0 |  |  |
Gain from dilutive effect restulitng from public offering |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.9 |
Net unrealized gains (losses) from Symetra's fixed maturity portfolio |  |  |  |  |  |  |  |  |  |  |  |  | 5.1 |  | 6.9 |  | (0.8) |
Loss from dilutive effect resulting from dividend and issuance of restricted shares |  |  |  |  |  |  |  |  |  |  |  |  | 1.3 |  | 1.0 |  | 0.8 |
Equity in net unrealized losses increase relating to amortization of common share impairment |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 136.6 |  |  |
Aggregate value of White Mountains common shares of Symetra at quoted market price | 225.9 |  |  |  |  |  |  |  |  |  |  |  | 225.9 |  |  |  |  |
Quoted market price (in dollars per share) | $ 12.98 |  |  |  |  |  |  |  |  |  |  |  | $ 12.98 |  |  |  |  |
Symetra | Common shareholders’ equity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value at the beginning of the period |  |  |  |  | 261.0 | [3] |  |  |  |  | 350.4 | [3] | 261.0 | [3] | 350.4 | [3] | 269.2 | [3]
Equity in earnings (losses) of unconsolidated affiliates |  |  |  |  |  |  |  |  |  |  |  |  | 32.3 | [4],[5],[6] | 28.2 | [4],[5] | 11.1 | [4],[5],[7]
Impairment of equity in earnings of Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (50.0) | [1] |  |
Equity in net unrealized losses from Symetra's fixed maturity portfolio |  |  |  |  |  |  |  |  |  |  |  |  | 62.8 | [8] | 85.0 | [8] | 72.7 | [8],[9]
Impairment of net unrealized gains from Symetra's fixed maturity portolio |  |  |  |  |  |  |  |  |  |  |  |  |  |  | (148.6) | [2] |  |
Dividends received |  |  |  |  |  |  |  |  |  |  |  |  | (4.9) |  | (4.0) |  | (2.6) |
Increase (decrease) in value of warrants |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  | 0 |  | 0 |
Carrying value at the end of the period | 351.2 | [10],[3] |  |  |  |  | 261.0 | [3] |  |  |  |  | 351.2 | [10],[3] | 261.0 | [3] | 350.4 | [3]
Equity in earnings increase relating to amortization of common share impairment |  |  |  |  |  |  |  |  |  |  |  |  | 3.5 |  |  |  |  |
Equity in net unrealized losses increase relating to amortization of common share impairment |  |  |  |  |  |  |  |  |  |  |  |  | 13.1 |  |  |  |  |
Symetra | Warrants |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value at the beginning of the period |  |  |  |  | 12.6 | [3] |  |  |  |  | 37.1 | [3] | 12.6 | [3] | 37.1 | [3] | 38.5 | [3]
Equity in earnings (losses) of unconsolidated affiliates |  |  |  |  |  |  |  |  |  |  |  |  | 0 | [4],[5],[6] | 0 | [4],[5] | 0 | [4],[5],[7]
Equity in net unrealized losses from Symetra's fixed maturity portfolio |  |  |  |  |  |  |  |  |  |  |  |  | 0 | [8] | 0 | [8] | 0 | [8],[9]
Dividends received |  |  |  |  |  |  |  |  |  |  |  |  | 0 |  | 0 |  | 0 |
Increase (decrease) in value of warrants |  |  |  |  |  |  |  |  |  |  |  |  | 17.7 |  | (24.5) |  | (1.4) |
Carrying value at the end of the period | $ 30.3 | [10],[3] |  |  |  |  | $ 12.6 | [3] |  |  |  |  | $ 30.3 | [10],[3] | $ 12.6 | [3] | $ 37.1 | [3]
[1] | Impairment of equity in earnings of Symetra excludes tax benefit of $4.1
[2] | Impairment of net unrealized gains from Symetra’s fixed maturity portfolio excludes tax benefit of $12.0
[3] | Includes White Mountains’ equity in net unrealized gains (losses) from Symetra’s fixed maturity portfolio of $62.8, $0, and $63.7 as of December 31, 2012, 2011 and 2010, which excludes tax expense of $5.1, $0 and $5.2
[4] | Equity in earnings for the years end December 31, 2012, 2011 and 2010 includes $1.3, $1.0, and $0.8 loss from the dilutive effect of Symetra’s yearly dividend and the issuance of restricted shares by Symetra
[5] | Equity in earnings for the years end December 31, 2012, 2011 and 2010 excludes tax expense of $2.6, $2.3, and $1.4
[6] | Equity in earnings includes $3.5 increase relating to the pre-tax amortization of Symetra common share impairment from December 31, 2012.
[7] | Includes a $17.9 loss from the dilutive effect of Symetra’s public offering
[8] | Net unrealized gains (losses) from Symetra’s fixed maturity portfolio excludes tax (expense) benefit of $(5.1), $(6.9) and $0.8 for the years ended December 31, 2012, 2011 and 2010.
[9] | Includes a $1.9 gain from the dilutive effect of Symetra’s public offering.
[10] | The aggregate value of White Mountains’ investment in common shares of Symetra was $225.9 based upon the quoted market price of $12.98 per share at December 31, 2012.

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