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Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Basic and diluted earnings per share numerators (in millions): |  |  |  |  |  |  |  |  |  |  |
Net income (loss) attributable to White Mountains’s common shareholders | $ 60.9 | $ 48.7 | $ 20.5 | $ 284.4 | $ (137.5) | $ 40.8 | $ 3.5 | $ (48.0) | $ 414.5 | $ (141.2) | $ 627.2
Less: total income (loss) from discontinued operations, net of tax |  |  |  |  |  |  |  |  | 0.8 | (17.2) | 577.5
Net income (loss) attributable to White Mountains’ common shareholders |  |  |  |  |  |  |  |  | 413.7 | (124.0) | 49.7
Allocation of (earnings) losses to participating restricted common shares |  |  |  |  |  |  |  |  | (5.3) | 1.4 | (0.7)
Basic and diluted earnings per share numerators |  |  |  |  |  |  |  |  | $ 408.4 | $ (122.6) | $ 49.0
Basic earnings per share denominators (in thousands): |  |  |  |  |  |  |  |  |  |  |
Weighted Average Number of Shares Issued, Basic |  |  |  |  |  |  |  |  | 3,181,600 | 3,382,500 | 4,293,800
Average unvested restricted common shares |  |  |  |  |  |  |  |  | (40,500) | (40,100) | (54,300)
Basic earnings (losses) per share denominator |  |  |  |  |  |  |  |  | 3,141,100 | 3,342,400 | 4,239,500
Diluted earnings per share denominator (in thousands): |  |  |  |  |  |  |  |  |  |  |
Average outstanding dilutive options to acquire common shares |  |  |  |  |  |  |  |  | 3,181,600 | 3,382,500 | 4,293,800
Average unvested restricted common shares |  |  |  |  |  |  |  |  | (40,500) | (40,100) | (54,300)
Diluted earnings (loss) per share denominator |  |  |  |  |  |  |  |  | 3,141,100 | 3,342,400 | 4,239,500
Diluted earnings per share (in dollars) - continuing operations: |  |  |  |  |  |  |  |  |  |  |
Dividends declared and paid |  |  |  |  |  |  |  |  | $ 1.00 | $ 1.00 | $ 1.00
Undistributed earnings (losses) |  |  |  |  |  |  |  |  | 129.02 | (37.67) | 10.56
Basic and diluted earnings (losses) per share |  |  |  |  |  |  |  |  | $ 130.02 | $ (36.67) | $ 11.56

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