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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 | 6 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 | Jun. 30, 2012 | Dec. 31, 2011 | Jun. 20, 2013
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |
Carrying value at the beginning of the period |  |  |  |  | $ 387.9 |  |  |  |  |
Equity in earnings | 7.1 |  | 6.5 |  | 16.3 |  | 16.7 |  |  |
Carrying value at the end of the period | 331.9 |  |  |  | 331.9 |  |  |  |  |
Symetra |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |
Carrying value at the beginning of the period | 377.5 |  | 293.2 |  | 381.5 |  | 273.6 |  |  |
Equity in earnings | 7.2 | [1],[2] | 7.0 | [1],[2] | 17.0 | [3],[4] | 17.9 | [3],[4] |  |
Equity in net unrealized losses from Symetra's fixed maturity portfolio | (71.8) | [5] | 30.6 | [5] | (80.5) | [6] | 29.4 | [6] |  |
Dividends received | (1.4) |  | (1.2) |  | (2.8) |  | (2.4) |  |  |
Increase (decrease) in value of warrants | (14.5) |  | (6.1) |  | (10.8) |  | (17.2) |  |  |
Exercise of warrants | 0 |  | 0 |  | 0 |  | 0 |  |  |
Carrying value at the end of the period | 326.0 | [7],[8],[9] | 335.7 | [7],[8],[9] | 326.0 | [7],[8],[9] | 335.7 | [7],[8],[9] | 273.6 |
Tax on equity in earnings | 0.6 |  | 0.6 |  | 1.4 |  | 1.5 |  |  |
Equity in net unrealized losses increase relating to amortization of common share impairment |  |  |  |  |  |  |  |  | 136.6 |
Tax on equity in net unrealized gains (losses) | 1.4 |  | (2.4) |  |  |  |  |  |  |
Aggregate value of White Mountains common shares of Symetra at quoted market price | 320.6 |  |  |  | 320.6 |  |  |  |  |
Market Price Per Share | $ 15.99 |  |  |  | $ 15.99 |  |  |  |  | $ 15.53
Symetra | Common shareholders' equity. |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |
Carrying value at the beginning of the period | 350.9 |  | 269.5 |  | 351.2 |  | 261.0 |  |  |
Equity in earnings | 7.2 | [1],[2] | 7.0 | [1],[2] | 17.0 | [3],[4] | 17.9 | [3],[4] |  |
Equity in net unrealized losses from Symetra's fixed maturity portfolio | (71.8) | [5] | 30.6 | [5] | (80.5) | [6] | 29.4 | [6] |  |
Dividends received | (1.4) |  | (1.2) |  | (2.8) |  | (2.4) |  |  |
Increase (decrease) in value of warrants | 0 |  | 0 |  | 0 |  | 0 |  |  |
Exercise of warrants | 41.1 |  | 0 |  | 41.1 |  | 0 |  |  |
Carrying value at the end of the period | 326.0 | [7],[8],[9] | 305.9 | [7],[8],[9] | 326.0 | [7],[8],[9] | 305.9 | [7],[8],[9] |  |
Impairment of Shares Change in Equity in Earnings | 0.8 |  | 0.7 |  | 1.5 |  | 1.8 |  |  |
Equity in net unrealized losses increase relating to amortization of common share impairment | 2.8 |  | 3.3 |  | 5.5 |  | 6.6 |  |  |
Symetra | Warrants |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |
Carrying value at the beginning of the period | 26.6 |  | 23.7 |  | 30.3 |  | 12.6 |  |  |
Equity in earnings | 0 | [1],[2] | 0 | [1],[2] | 0 | [3],[4] | 0 | [3],[4] |  |
Equity in net unrealized losses from Symetra's fixed maturity portfolio | 0 | [5] | 0 | [5] | 0 | [6] | 0 | [6] |  |
Dividends received | 0 |  | 0 |  | 0 |  | 0 |  |  |
Increase (decrease) in value of warrants | (14.5) |  | (6.1) |  | (10.8) |  | (17.2) |  |  |
Exercise of warrants | (41.1) |  | 0 |  | (41.1) |  | 0 |  |  |
Carrying value at the end of the period | 0 | [7],[8],[9] | 29.8 | [7],[8],[9] | 0 | [7],[8],[9] | 29.8 | [7],[8],[9] |  |
White Mountains |  |  |  |  |  |  |  |  |  |
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |
Equity in net unrealized losses from Symetra's fixed maturity portfolio | (17.8) |  | 29.4 |  | (17.8) |  | 29.4 |  |  |
Tax on equity in net unrealized gains (losses) |  |  |  |  | $ 1.4 |  | $ (2.4) |  |  |
[1] | Equity in earnings excludes tax expense of $0.6 and $0.6.
[2] | Equity in earnings includes $0.8 and $0.7 increase relating to the pre-tax amortization of the Symetra common share impairment.
[3] | Equity in earnings excludes tax expense of $1.4 and $1.5.
[4] | Equity in earnings includes $1.5 and $1.8 increase relating to the pre-tax amortization of the Symetra common share impairment.
[5] | Net unrealized gains includes $2.8 and $3.3 increase relating to the pre-tax amortization of the Symetra common share impairmen
[6] | Net unrealized gains includes $5.5 and $6.6 increase relating to the pre-tax amortization of the Symetra common share impairment.
[7] | 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.
[8] | 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).
[9] | 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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