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Discontinued Operations (Details 3) (USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2011 | Sep. 30, 2011 | Jun. 30, 2011 | Mar. 31, 2011 | Dec. 31, 2010 | Sep. 30, 2010 | Jun. 30, 2010 | Mar. 31, 2010 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
Basic and diluted earnings per share numerators (in millions): |  |  |  |  |  |  |  |  |  |  |
Net income (loss) attributable to White Mountains' common shareholders | $ 674.2 | $ (30.1) | $ (1.5) | $ 2.5 | $ (7.8) | $ 3.6 | $ 0.6 | $ (3.0) | $ (13.0) | $ (6.6) | $ (27.8)
Allocation of income for participating unvested restricted common shares |  |  |  |  |  |  |  |  | 1.1 | 1.1 | 4.8
Net income (loss) attributable to White Mountains' common shareholders, net of restricted common share amounts | 800.2 | (15.8) | 11.7 | (28.2) | 72.9 | 50.7 | 2.5 | (39.6) | 767.9 | 86.5 | 470.0
Basic earnings per share denominators (in thousands): |  |  |  |  |  |  |  |  |  |  |
Total average common shares outstanding during the period |  |  |  |  |  |  |  |  | 7,881,000 | 8,548,400 | 8,849,400
Average unvested restricted common shares |  |  |  |  |  |  |  |  | (69,400) | (97,300) | (85,300)
Basic earnings per share denominator (in shares) |  |  |  |  |  |  |  |  | 7,811,600 | 8,451,100 | 8,764,100
Diluted earnings per share denominator (in thousands): |  |  |  |  |  |  |  |  |  |  |
Total average common shares outstanding during the period |  |  |  |  |  |  |  |  | 7,881,000 | 8,548,400 | 8,849,400
Average unvested restricted common shares |  |  |  |  |  |  |  |  | (69,400) | (97,300) | (85,300)
Average outstanding dilutive options to acquire common shares (in shares) |  |  |  |  |  |  |  |  |  | 500 | 1,500
Diluted earnings per share denominator (in shares) |  |  |  |  |  |  |  |  | 7,811,600 | 8,451,600 | 8,765,600
Common shares issuable upon exercise of incentive options (in shares) |  |  |  |  |  |  |  |  |  | 1,200 | 5,280
Average strike price of options (in dollars per share) |  |  |  |  | $ 189.31 |  |  |  |  | $ 189.31 | $ 183.03
Discontinued Operations. |  |  |  |  |  |  |  |  |  |  |
Basic and diluted earnings per share numerators (in millions): |  |  |  |  |  |  |  |  |  |  |
Net income (loss) attributable to White Mountains' common shareholders |  |  |  |  |  |  |  |  | 645.3 | (6.6) | (27.8)
Allocation of income for participating unvested restricted common shares |  |  |  |  |  |  |  |  | (5.7) | 0.1 | 0.3
Net income (loss) attributable to White Mountains' common shareholders, net of restricted common share amounts |  |  |  |  |  |  |  |  | $ 639.5 | $ (6.5) | $ (27.5)
Basic earnings per share denominators (in thousands): |  |  |  |  |  |  |  |  |  |  |
Total average common shares outstanding during the period |  |  |  |  |  |  |  |  | 7,881,000 | 8,548,400 | 8,849,400
Average unvested restricted common shares |  |  |  |  |  |  |  |  | (69,400) | (97,300) | (85,300)
Basic earnings per share denominator (in shares) |  |  |  |  |  |  |  |  | 7,811,600 | 8,451,100 | 8,764,100
Diluted earnings per share denominator (in thousands): |  |  |  |  |  |  |  |  |  |  |
Total average common shares outstanding during the period |  |  |  |  |  |  |  |  | 7,881,000 | 8,548,400 | 8,849,400
Average unvested restricted common shares |  |  |  |  |  |  |  |  | (69,400) | (97,300) | (85,300)
Average outstanding dilutive options to acquire common shares (in shares) |  |  |  |  |  |  |  |  |  | 500 | 1,500
Diluted earnings per share denominator (in shares) |  |  |  |  |  |  |  |  | 7,811,600 | 8,451,600 | 8,765,600
Basic and diluted earnings (loss) per share (in dollars): |  |  |  |  |  |  |  |  | $ 81.88 | $ (0.77) | $ (3.14)

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