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Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2016 | Sep. 30, 2015
Basic and diluted earnings per share numerators (in millions): |  |  |  |
Net income (loss) attributable to White Mountains’ common shareholders | $ 29.8 | $ (65.4) | $ 24.2 | $ (47.6)
Allocation of income for unvested restricted common shares | (0.4) | 0.0 | (0.3) | 0.0
Dividends declared on participating restricted common shares | 0.0 | 0.0 | (0.1) | (0.1)
Total allocation to restricted common shares | (0.4) | 0.0 | (0.4) | (0.1)
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts | 29.4 | (65.4) | 23.8 | (47.7)
Undistributed net earnings (in millions): |  |  |  |
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts | 29.4 | (65.4) | 23.8 | (47.7)
Dividends declared net of restricted common share amounts | 0.0 | 0.0 | (5.4) | (5.9)
Total undistributed net earnings, net of restricted common share amounts | $ 29.4 | $ (65.4) | $ 18.4 | $ (53.6)
Basic earnings per share denominators |  |  |  |
Total average common shares outstanding during the period | 4,867,400 | 5,890,100 | 5,166,600 | 5,951,100
Average unvested restricted common shares | (68,100) | 0 | (62,900) | 0
Basic earnings per share denominator | 4,799,300 | 5,890,100 | 5,103,700 | 5,951,100
Diluted earnings per share denominator |  |  |  |
Total average common shares outstanding during the period | 4,867,400 | 5,890,100 | 5,166,600 | 5,951,100
Average unvested restricted common shares | (68,100) | 0 | (62,900) | 0
Average outstanding dilutive options to acquire common shares | 4,879,400 | 5,890,100 | 5,174,800 | 5,951,100
Diluted earnings (loss) per share denominator | 4,811,300 | 5,890,100 | 5,111,900 | 5,951,100
Basic earnings per share (in dollars): |  |  |  |
Continuing operations | $ 6.12 | $ (11.10) | $ 4.67 | $ (8.01)
Undistributed earnings (loss) (in dollars per share) | 6.12 | (11.10) | 3.67 | (9.01)
Earnings Per Share, Diluted [Abstract] |  |  |  |
Continuing operations | 6.11 | (11.10) | 4.66 | (8.01)
Dividends declared | (0.00) | (0.00) | (1.00) | (1.00)
Undistributed earnings (loss) (in dollars per share) | $ 6.11 | $ (11.10) | $ 3.66 | $ (9.01)
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements | 11,943 |  | 8,208 |
Discontinued Operations [Member] |  |  |  |
Basic and diluted earnings per share numerators (in millions): |  |  |  |
Allocation of income for unvested restricted common shares | $ 0.9 | $ 0.1 | $ (5.1) | $ 0.9
Basic earnings per share denominators |  |  |  |
Total average common shares outstanding during the period | 4,879,400 | 5,890,100 | 5,174,800 | 5,951,100
Average unvested restricted common shares | (68,100) | (70,900) | (62,900) | (67,100)
Basic earnings per share denominator | 4,799,300 | 5,819,200 | 5,103,700 | 5,884,000
Diluted earnings per share denominator |  |  |  |
Total average common shares outstanding during the period | 4,879,400 | 5,890,100 | 5,174,800 | 5,951,100
Average unvested restricted common shares | (68,100) | (70,900) | (62,900) | (67,100)
Diluted earnings (loss) per share denominator | 4,811,300 | 5,819,200 | 5,111,900 | 5,884,000
Employee Stock Option [Member] |  |  |  |
Earnings Per Share, Diluted [Abstract] |  |  |  |
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements | 120,000 |  | 120,000 |

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- Definition The earnings attributable to each security on the basis that all of the earnings for the period had been distributed (to the extent that each security may share in earnings). This is the earnings amount which reflects the participation rights in undistributed earnings of each security. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 66 -URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e2814-109256 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 60 -URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e2740-109256 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 65 -URI http://asc.fasb.org/extlink&oid=7655603&loc=d3e2793-109256 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 60A -URI http://asc.fasb.org/extlink&oid=7655603&loc=SL5780132-109256 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 60B -URI http://asc.fasb.org/extlink&oid=7655603&loc=SL5780133-109256 + Details
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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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