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Earnings (Loss) Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended
--- | ---
Mar. 31, 2012 | Mar. 31, 2011
Basic and diluted earnings per share numerators (in millions): |  |
Net income (loss) attributable to White Mountains' common shareholders | $ 93.7 | $ (30.7)
Allocation of income for participating unvested restricted common shares | (1.0) | 0.2
Dividends declared on participating restricted common shares | (0.1) | (0.1)
Total allocation to restricted common shares | (1.1) | 0.1
Net income (loss) attributable to White Mountains' common shareholders, net of restricted share amounts | 92.6 | (30.6)
Undistributed net earnings (in millions): |  |
Net income (loss) attributable to White Mountains' common shareholders, net of restricted common share amounts | 92.6 | (30.6)
Dividends declared net of restricted common share amounts | (6.6) | (7.9)
Total undistributed net earnings (loss), net of restricted common share amounts | $ 86.0 | $ (38.5)
Basic earnings per share denominators (in thousands): |  |
Total average common shares outstanding during the period | 7,431,700 | 8,031,800
Average unvested restricted common shares | (78,300) | (57,100)
Basic earnings (loss) per share denominator | 7,353,400 | 7,974,700
Diluted earnings per share denominator (in thousands): |  |
Total average common shares outstanding during the period | 7,431,700 | 8,031,800
Average unvested restricted common shares | (78,300) | (57,100)
Diluted earnings (loss) per share denominator | 7,353,400 | 7,974,700
Basic earnings per share (in dollars): |  |
Net income (loss) attributable to White Mountains' common shareholders | $ 12.59 | $ (3.82)
Dividends declared (in dollars per share) | $ (1.00) | $ (1.00)
Undistributed earnings (loss) (in dollars per share) | $ 11.59 | $ (4.82)
Diluted earnings per share (in dollars) |  |
Net income (loss) attributable to White Mountains' common shareholders (in dollars per share) | $ 12.59 | $ (3.82)
Dividends declared (in dollars per share) | $ (1.00) | $ (1.00)
Undistributed earnings (loss) (in dollars per share) | $ 11.59 | $ (4.82)

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