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Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
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
Quarterly Financial Information Disclosure [Abstract] |  |  |  |  |  |  |  |  |  |  |
Revenues (including realized gains and losses) | $ 626.6 | $ 574.1 | $ 489.3 | $ 627.4 | $ 566.4 | $ 697.4 | $ 542.3 | $ 629.7 | $ 2,317.4 | $ 2,435.7 | $ 2,173.1
Total expenses | 492.4 | 518.8 | 485.7 | 475.6 | 617.2 | 562.3 | 504.7 | 488.8 | 1,972.5 | 2,172.9 | 2,075.1
Pre-tax loss | 134.2 | 55.3 | 3.6 | 151.8 | (50.8) | 135.1 | 37.6 | 140.9 | 344.9 | 262.8 | 98.0
Tax benefit (expense) | (27.4) | (8.2) | 0.6 | (41.6) | 101.0 | (47.8) | (6.4) | (31.1) | (76.6) | 15.7 | 110.0
Net income from continuing operations | 106.8 | 47.1 | 4.2 | 110.2 | 50.2 | 87.3 | 31.2 | 109.8 | 268.3 | 278.5 | 208.0
Net loss from discontinued operations, net of tax | (0.3) | 0.4 | 3.9 | 0.5 | 0.5 | (106.8) | 0.5 | (9.2) | (42.1) | (24.0) | (36.7)
Non-controlling interest in consolidated subsidiaries | (0.3) | 1.1 | 11.1 | 0.5 | 12.0 | 30.9 | (12.1) | (16.8) | 12.4 | 14.0 | (41.5)
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)
Net income attributable to White Mountains’ common shareholders | $ 117.9 | $ 57.2 | $ 26.3 | $ 120.4 | $ 68.2 | $ 19.1 | $ 26.1 | $ 94.0 | $ 321.8 | $ 207.4 | $ 767.9
Basic earnings (loss) per share |  |  |  |  |  |  |  |  |  |  |
Continuing operations | $ 19.14 | $ 9.20 | $ 3.62 | $ 19.10 | $ 10.36 | $ 19.11 | $ 3.85 | $ 13.85 | $ 51.15 | $ 47.41 | $ 18.56
Discontinued operations | $ (0.05) | $ 0.06 | $ 0.64 | $ 0.07 | $ 0.07 | $ (16.21) | $ 0.07 | $ (1.24) | $ 0.74 | $ (16.91) | $ 78.88
Total consolidated operations | $ 19.09 | $ 9.26 | $ 4.26 | $ 19.17 | $ 10.43 | $ 2.90 | $ 3.92 | $ 12.61 | $ 51.89 | $ 30.50 | $ 97.44
Diluted earnings (loss) per share |  |  |  |  |  |  |  |  |  |  |
Continuing operations | $ 19.14 | $ 9.20 | $ 3.62 | $ 19.10 | $ 10.36 | $ 19.11 | $ 3.85 | $ 13.85 | $ 51.15 | $ 47.41 | $ 18.56
Discontinued operations | $ (0.05) | $ 0.06 | $ 0.64 | $ 0.07 | $ 0.07 | $ (16.21) | $ 0.07 | $ (1.24) | $ 0.74 | $ (16.91) | $ 78.88
Total consolidated operations | $ 19.09 | $ 9.26 | $ 4.26 | $ 19.17 | $ 10.43 | $ 2.90 | $ 3.92 | $ 12.61 | $ 51.89 | $ 30.50 | $ 97.44
Adjusted book value per share | $ 642.27 | $ 621.56 | $ 604.75 | $ 606.20 | $ 587.63 | $ 573.66 | $ 564.77 | $ 565.38 |  |  |

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