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Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 6 Months Ended | 9 Months Ended
--- | --- | --- | ---
Sep. 30, 2015 | Sep. 30, 2014 | Jun. 30, 2014 | Sep. 30, 2015 | Sep. 30, 2014
Basic and diluted earnings per share numerators (in millions): |  |  |  |  |
Net income (loss) attributable to White Mountains’ common shareholders | $ (73.1) | $ (12.6) |  | $ (61.8) | $ 65.8
Allocation of income for unvested restricted common shares | 0.0 | 0.0 |  | 0.0 | (0.8)
Dividends declared on participating restricted common shares | 0.0 | 0.0 |  | (0.1) | (0.1)
Total allocation to restricted common shares | 0.0 | 0.0 |  | (0.1) | (0.9)
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts | (73.1) | (12.6) |  | (61.9) | 64.9
Undistributed net earnings (in millions): |  |  |  |  |
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts | (73.1) | (12.6) |  | (61.9) | 64.9
Dividends declared net of restricted common share amounts | 0.0 | 0.0 |  | (5.9) | (6.1)
Total undistributed net earnings, net of restricted common share amounts | $ (73.1) | $ (12.6) |  | $ (67.8) | $ 58.8
Basic earnings per share denominators |  |  |  |  |
Total average common shares outstanding during the period | 5,890,100 | 6,091,400 |  | 5,951,100 | 6,140,900
Average unvested restricted common shares | 0 | 0 |  | 0 | (77,700)
Basic earnings per share denominator | 5,890,100 | 6,091,400 |  | 5,951,100 | 6,063,200
Diluted earnings per share denominator |  |  |  |  |
Total average common shares outstanding during the period | 5,890,100 | 6,091,400 |  | 5,951,100 | 6,140,900
Average unvested restricted common shares | 0 | 0 |  | 0 | (77,700)
Average outstanding dilutive options to acquire common shares | 0 | 0 |  | 0 | 0
Diluted earnings (loss) per share denominator | 5,890,100 | 6,091,400 |  | 5,951,100 | 6,063,200
Basic earnings per share (in dollars): |  |  |  |  |
Continuing operations | $ (12.42) | $ (2.07) |  | $ (10.40) | $ 10.70
Dividends declared | (1.00) | (1.00) |  | (1.00) | (1.00)
Undistributed earnings (loss) (in dollars per share) | (12.42) | (2.07) |  | (11.40) | 9.70
Earnings Per Share, Diluted [Abstract] |  |  |  |  |
Continuing operations | (12.42) | (2.07) |  | (10.40) | 10.70
Dividends declared | (1.00) | (1.00) |  | (1.00) | (1.00)
Undistributed earnings (loss) (in dollars per share) | $ (12.42) | $ (2.07) |  | $ (11.40) | $ 9.70
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements | 125,000 | 125,000 |  | 125,000 | 125,000
Discontinued Operations [Member] |  |  |  |  |
Basic and diluted earnings per share numerators (in millions): |  |  |  |  |
Allocation of income for unvested restricted common shares | $ 0.2 | $ (0.9) |  | $ (1.0) | $ 2.2
Basic earnings per share denominators |  |  |  |  |
Total average common shares outstanding during the period | 5,890,100 | 6,091,400 |  | 5,951,100 | 6,140,900
Average unvested restricted common shares | (70,900) | (81,300) |  | (67,100) | (77,700)
Basic earnings per share denominator | 5,819,200 | 6,010,100 |  | 5,884,000 | 6,063,200
Diluted earnings per share denominator |  |  |  |  |
Total average common shares outstanding during the period | 5,890,100 | 6,091,400 |  | 5,951,100 | 6,140,900
Average unvested restricted common shares | (70,900) | (81,300) |  | (67,100) | (77,700)
Average outstanding dilutive options to acquire common shares | 0 | 0 |  | 0 | 0
Diluted earnings (loss) per share denominator | 5,819,200 | 6,010,100 |  | 5,884,000 | 6,063,200
Earnings Per Share, Diluted [Abstract] |  |  |  |  |
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements |  |  | 125,000 |  |

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- Definition Number of shares of restricted stock determined by relating the portion of time within a reporting period that restricted shares have been outstanding to the total time in that period. Restricted shares are subject to sales, contractual, regulatory or other restrictions that prevent or inhibit the holder from freely disposing of them before the restriction ends. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e2646-109256 + Details
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- Definition Represents Net income from continuing operations attributable to White Mountains common shareholders, net of allocation to restricted shares and dividends paid on restricted shares. + References No definition available. + Details
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