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Earnings Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2013 | Jun. 30, 2012 | Jun. 30, 2013 | Jun. 30, 2012
Basic and diluted earnings per share numerators (in millions): |  |  |  |  |  |  |  |
Net income (loss) attributable to White Mountains’ common shareholders | $ 22.4 |  | $ 25.6 |  | $ 142.3 |  | $ 128.7 |
Allocation of income for unvested restricted common shares | (0.4) |  | (0.4) |  | (2.0) |  | (1.6) |
Dividends declared on participating restricted common shares | 0 | [1] | 0 | [1] | (0.1) | [1] | (0.1) | [1]
Total allocation to restricted common shares | (0.4) |  | (0.4) |  | (2.1) |  | (1.7) |
Net income attributable to White Mountains’ common shareholders, net of restricted common share amounts | 22.0 |  | 25.2 |  | 140.2 |  | 127.0 |
Undistributed net earnings (in millions): |  |  |  |  |  |  |  |
Net income attributable to White Mountains’ common shareholders, net of restricted common share amounts | 22.0 |  | 25.2 |  | 140.2 |  | 127.0 |
Dividends declared net of restricted common share amounts | 0 | [1] | 0 | [1] | (6.1) | [1] | (6.5) | [1]
Total undistributed net earnings, net of restricted common share amounts | $ 22.0 |  | $ 25.2 |  | $ 134.1 |  | $ 120.5 |
Basic earnings per share denominators (in thousands): |  |  |  |  |  |  |  |
Total average common shares outstanding during the period | 6,176,200 |  | 6,638,700 |  | 6,224,500 |  | 7,033,000 |
Average unvested restricted common shares | (95,400) | [2],[3] | (93,500) | [2],[4] | (87,200) | [2],[3] | (85,900) | [2]
Basic earnings per share denominator | 6,080,800 |  | 6,545,200 |  | 6,137,300 |  | 6,947,100 |
Diluted earnings per share denominator (in thousands): |  |  |  |  |  |  |  |
Total average common shares outstanding during the period | 6,176,200 |  | 6,638,700 |  | 6,224,500 |  | 7,033,000 |
Average unvested restricted common shares | (95,400) | [2],[3] | (93,500) | [2],[4] | (87,200) | [2],[3] | (85,900) | [2]
Average outstanding dilutive options to acquire common shares | 0 | [3] | 0 | [3] | 0 | [3] | 0 | [3]
Diluted earnings (loss) per share denominator | 6,080,800 |  | 6,545,200 |  | 6,137,300 |  | 6,947,100 |
Basic earnings per share (in dollars): |  |  |  |  |  |  |  |
Net income (loss) attributable to White Mountains' common shareholders | $ 3.62 |  | $ 3.85 |  | $ 22.84 |  | $ 18.28 |
Dividends declared | $ 0 |  | $ 0 |  | $ (1) |  | $ (1) |
Undistributed earnings (loss) (in dollars per share) | $ 3.62 |  | $ 3.85 |  | $ 21.84 |  | $ 17.28 |
Earnings Per Share, Diluted [Abstract] |  |  |  |  |  |  |  |
Net income (loss) attributable to White Mountains' common shareholders (in dollars per share) | $ 3.62 |  | $ 3.85 |  | $ 22.84 |  | $ 18.28 |
Dividends declared | $ 0 |  | $ 0 |  | $ (1) |  | $ (1) |
Undistributed earnings (loss) (in dollars per share) | $ 3.62 |  | $ 3.85 |  | $ 21.84 |  | $ 17.28 |
[1] | Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.
[2] | Restricted shares outstanding vest either in equal annual installments or upon a stated date (see Note 13).
[3] | The diluted earnings per share denominator for the three and six months ended June 30, 2013 and 2012 do not include common shares issuable upon exercise of the Non-Qualified Options as they are anti-dilutive to the calculation.
[4] | Restricted common shares outstanding vest either in equal annual installments or upon a stated date (see Note 13).

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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 Emerging Issues Task Force (EITF) -Number 03-6 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 128 -Paragraph 61 -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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