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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 | 3 Months Ended | 6 Months Ended | 12 Months Ended |  | 12 Months Ended
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Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2012 | Sep. 30, 2012 | Jun. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2013 Symetra | Dec. 31, 2012 Symetra | Dec. 31, 2011 Symetra | Dec. 31, 2013 Common Stock Symetra | Dec. 31, 2012 Common Stock Symetra | Dec. 31, 2011 Common Stock Symetra | Dec. 31, 2013 Common Stock White Mountains | Dec. 31, 2012 Common Stock White Mountains | Dec. 31, 2011 Common Stock White Mountains | Jun. 30, 2013 Warrants | Jun. 20, 2013 Warrants | Dec. 31, 2013 Warrants Symetra | Dec. 31, 2012 Warrants Symetra | Dec. 31, 2011 Warrants Symetra | Jun. 20, 2013 Warrants Symetra | Dec. 31, 2013 equity in earnings of unconsolidated affiliates [Member] Common Stock Symetra | Dec. 31, 2012 equity in earnings of unconsolidated affiliates [Member] Common Stock Symetra | Dec. 31, 2011 equity in earnings of unconsolidated affiliates [Member] Common Stock Symetra | Dec. 31, 2013 Net unrealized gains (losses) of unconsolidated affiliates [Member] Common Stock Symetra | Dec. 31, 2012 Net unrealized gains (losses) of unconsolidated affiliates [Member] Common Stock Symetra | Dec. 31, 2011 Net unrealized gains (losses) of unconsolidated affiliates [Member] Common Stock Symetra | Dec. 31, 2013 Net unrealized gains (losses) of unconsolidated affiliates [Member] Common Stock White Mountains | Dec. 31, 2012 Net unrealized gains (losses) of unconsolidated affiliates [Member] Common Stock White Mountains | Dec. 31, 2011 Net unrealized gains (losses) of unconsolidated affiliates [Member] Common Stock White Mountains | Dec. 31, 2011 impairment of equity in earnings [Member] equity in earnings of unconsolidated affiliates [Member] Common Stock Symetra | Dec. 31, 2013 impairment of net unrealized gains (losses) [Member] Net unrealized gains (losses) of unconsolidated affiliates [Member] Common Stock Symetra | Dec. 31, 2012 impairment of net unrealized gains (losses) [Member] Net unrealized gains (losses) of unconsolidated affiliates [Member] Common Stock Symetra | Dec. 31, 2011 impairment of net unrealized gains (losses) [Member] Net unrealized gains (losses) of unconsolidated affiliates [Member] Common Stock Symetra | Dec. 31, 2013 Amortization of common share basis difference [Member] Common Stock Symetra | Dec. 31, 2012 Amortization of common share basis difference [Member] Common Stock Symetra
Changes in carrying value of investment in Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value at the beginning of the period |  |  |  | $ 387.9 |  |  |  |  | $ 387.9 |  |  | $ 381.5 | [1] | $ 273.6 | [1] | $ 387.5 |  | $ 351.2 | [1] | $ 261.0 | [1] | $ 350.4 |  |  |  |  |  |  | $ 30.3 | [1] | $ 12.6 | [1] | $ 37.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Equity in earnings (losses) of unconsolidated affiliates, net of tax | 11.7 | 8.6 | 7.1 | 9.2 | 5.5 | 7.7 | 6.5 | 10.2 | 36.6 | 29.9 | (20.2) | 37.8 |  | 32.3 | [2],[3],[4] | 28.2 | [2],[3] | 37.8 |  | 32.3 | [2],[3],[4] | 28.2 | [2],[3] |  |  |  |  |  | 0 |  | 0 | [2],[3],[4] | 0 | [2],[3] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 3.0 | 3.5
Impairment of equity in earnings of Symetra |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0 | [5] |  |  |  | (50.0) | [6] |  |  | (148.6) | [5] |  |  |  |  |  |  |  |  |
Net change in pre-tax unrealized gains (losses) on investments in unconsolidated affiliates |  |  |  |  |  |  |  |  | (106.4) | 62.8 | (63.6) | (106.4) | [7],[8] | 62.8 | [7],[8] | 85.0 | [7] | (106.4) | [7],[8] | 62.8 | [7],[8] | 85.0 | [7] | (43.6) | 62.8 | 0 |  |  | 0 | [7],[8] | 0 | [7],[8] | 0 | [7] |  |  |  |  |  |  |  |  |  |  |  |  |  | 11.8 | 13.1 |  |  |
Dividends received |  |  |  |  |  |  |  |  |  |  |  | (6.4) |  | (4.9) |  | (4.0) |  | (6.4) |  | (4.9) |  | (4.0) |  |  |  |  |  |  | 0 |  | 0 |  | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Increase (decrease) in value of warrants |  |  |  |  |  |  |  |  |  |  |  | 10.8 |  | 17.7 |  | (24.5) |  | 0 |  | 0 |  | 0 |  |  |  |  | 14.5 | 10.8 | 10.8 |  | 17.7 |  | (24.5) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Carrying value at the end of the period | 321.4 |  |  |  | 387.9 |  |  |  | 321.4 | 387.9 |  | 317.3 | [2],[9] | 381.5 | [1] | 273.6 | [1] | 317.3 | [2],[9] | 351.2 | [1] | 261.0 | [1] |  |  |  |  |  | 0 | [2],[9] | 30.3 | [1] | 12.6 | [1] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unrealized (losses) gains from Symetra’s fixed maturity portfolio |  |  |  |  |  |  |  |  |  |  |  | (43.6) |  | 62.8 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Income Tax Expense (Benefit) | 27.4 | 8.2 | (0.6) | 41.6 | (101.0) | 47.8 | 6.4 | 31.1 | 76.6 | (15.7) | (110.0) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.8 | 2.6 | 2.3 |  | (8.3) | 5.1 | 6.9 |  | (3.2) | 5.1 | 0 | (4.1) |  |  | (12.0) |  |
Loss from dilutive effect resulting from dividend and issuance of restricted shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 0.2 |  | 1.3 |  | 1.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 380.1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share Price |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 18.96 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 15.53 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Exercise of warrants |  |  |  |  |  |  |  |  |  |  |  | $ 0 |  |  |  |  |  | $ 41.1 |  |  |  |  |  |  |  |  |  |  | $ (41.1) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | Includes White Mountains’ equity in net unrealized (losses) gains from Symetra’s fixed maturity portfolio of $(43.6), $62.8, and $0 as of December 31, 2013, 2012 and 2011, which excludes tax benefit (expense) of $3.2, $(5.1) and $0
[2] | Equity in earnings for the years end December 31, 2013, 2012 and 2011 excludes tax expense of $2.8, $2.6, and $2.3
[3] | Equity in earnings for the years end December 31, 2013, 2012 and 2011 includes $.2, $1.3, and $1.0 loss from the dilutive effect of Symetra’s yearly dividend and the issuance of restricted shares by Symetra
[4] | Equity in earnings for the years ended December 31, 2013 and 2012 includes $3.0 and $3.5 increases relating to the pre-tax amortization of the Symetra common share basis difference.
[5] | Impairment of net unrealized gains from Symetra’s fixed maturity portfolio excludes tax benefit $12.0
[6] | Impairment of equity in earnings of Symetra excludes tax benefit of $4.1
[7] | Net unrealized (losses) gains from Symetra’s fixed maturity portfolio excludes tax benefit (expense) of $8.3, $(5.1) and $(6.9) for the years ended December 31, 2013, 2012 and 2011.
[8] | Net unrealized gains for the years ended December 31, 2013 and 2012 includes $11.8 and $13.1 increases relating to the pre-tax amortization of the Symetra common share basis difference.
[9] | The aggregate value of White Mountains’ investment in common shares of Symetra was $380.1 based upon the quoted market price of $18.96 per share at December 31, 2013.

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