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Discontinued Operations (Earnings Per Share) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended | 12 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Mar. 31, 2013 | Dec. 31, 2012 | Sep. 30, 2012 | Jun. 30, 2012 | Mar. 31, 2012 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Sep. 30, 2013 Discontinued Operations | Dec. 31, 2013 Discontinued Operations | Dec. 31, 2012 Discontinued Operations | Dec. 31, 2011 Discontinued Operations
Discontinued Operations |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Net income (loss) from discontinued operations |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 4.5 |  | $ (115.0) |  | $ 621.6 |
Basic and diluted earnings (loss) per share numerators (in millions): |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Allocation of income for participating unvested restricted common shares |  |  |  |  |  |  |  |  | (4.7) |  | (4.3) |  | (1.2) |  |  | 0 | [1] | 1.5 | [1] | (5.4) | [1]
Net income attributable to White Mountains’ common shareholders | $ 117.9 | $ 57.2 | $ 26.3 | $ 120.4 | $ 68.2 | $ 19.1 | $ 26.1 | $ 94.0 | $ 321.8 |  | $ 207.4 |  | $ 767.9 |  |  | $ 4.5 |  | $ (113.5) |  | $ 616.2 |
Basic earnings (loss) per share denominators (in thousands): |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total average common shares outstanding during the period |  |  |  |  |  |  |  |  | 6,200,400 |  | 6,799,800 |  | 7,881,000 |  |  | 6,200,400 |  | 6,799,800 |  | 7,881,000 |
Average unvested restricted common shares |  |  |  |  |  |  |  |  | (91,400) | [1] | (91,100) | [1] | (69,400) | [1] |  | (91,400) | [1] | (91,100) | [1] | (69,400) | [1]
Basic earnings (loss) per share denominator |  |  |  |  |  |  |  |  | 6,109,000 |  | 6,708,700 |  | 7,811,600 |  |  | 6,109,000 |  | 6,708,700 |  | 7,811,600 |
Diluted earnings (loss) per share denominator (in thousands): |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total average common shares outstanding during the period |  |  |  |  |  |  |  |  | 6,200,400 |  | 6,799,800 |  | 7,881,000 |  |  | 6,200,400 |  | 6,799,800 |  | 7,881,000 |
Average unvested restricted common shares |  |  |  |  |  |  |  |  | (91,400) | [1] | (91,100) | [1] | (69,400) | [1] |  | (91,400) | [1] | (91,100) | [1] | (69,400) | [1]
Average outstanding dilutive options to acquire common shares |  |  |  |  |  |  |  |  | 0 | [2] | 0 | [2] | 0 | [2] |  | 0 | [2] | 0 | [2] | 0 | [2]
Diluted earnings (loss) per share denominator |  |  |  |  |  |  |  |  | 6,109,000 |  | 6,708,700 |  | 7,811,600 |  |  | 6,109,000 |  | 6,708,700 |  | 7,811,600 |
Basic and diluted earnings (loss) per share (in dollars): |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.74 |  | $ (16.91) |  | $ 78.88 |
Incremental common shares attributable to share-based payment arrangements |  |  |  |  |  |  |  |  | 125,000 |  |  |  |  |  | 125,000 |  |  |  |  |  |
[1] | Restricted common shares outstanding vest either in equal annual installments or upon a stated date (see Note 12).
[2] | The diluted earnings per share denominator for the years ended December 31, 2013, 2012 and 2011 do not include the impact of 125,000 common shares issuable upon exercise of the non-qualified options outstanding as they are anti-dilutive to the calculation.

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- Definition The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period. + References No definition available. + Details
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- Definition Amount of income (loss) from a disposal group, net of income tax, reported as a separate component of income before extraordinary items after deduction or consideration of the amount allocable to noncontrolling interests. Includes, net of tax, income (loss) from operations during the phase-out period, gain (loss) on disposal, provision (or any reversals of earlier provisions) for loss on disposal, and adjustments of a prior period gain (loss) on disposal. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 45 -Paragraph 19 -URI http://asc.fasb.org/extlink&oid=7656940&loc=SL4569616-111683 + 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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