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Earnings Per Share - Schedule of Earnings Per Share from Continuing Operations (Details) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
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Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Basic and diluted earnings per share numerators (in millions): |  |  |
Net income (loss) attributable to White Mountains’s common shareholders | $ (275.4) | $ 708.7 | $ 414.5
Less: total (loss) income from discontinued operations, net of tax | 18.7 | (2.3) | 0.8
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders | (294.1) | 711.0 | 413.7
Allocation of (earnings) losses to participating restricted common shares | 3.5 | (9.3) | (5.3)
Basic and diluted earnings (losses) per share numerators | $ (290.6) | $ 701.7 | $ 408.4
Basic earnings per share denominators (in thousands): |  |  |
Total average common shares outstanding during the period (in shares) | 3,079,000 | 3,122,200 | 3,181,600
Average unvested restricted common shares (in shares) | (36,500) | (40,800) | (40,500)
Basic earnings (losses) per share denominator (in shares) | 3,042,500 | 3,081,400 | 3,141,100
Diluted earnings per share denominator (in thousands): |  |  |
Average outstanding dilutive options to acquire common shares (in shares) | 3,079,000 | 3,122,200 | 3,181,600
Average unvested restricted common shares (in shares) | (36,500) | (40,800) | (40,500)
Diluted earnings (loss) per share denominator (in shares) | 3,042,500 | 3,081,400 | 3,141,100
Diluted earnings per share (in dollars) - continuing operations: |  |  |
Dividends declared and paid (usd per share) | $ 1.00 | $ 1.00 | $ 1.00
Undistributed earnings (losses) (usd per share) | (96.53) | 226.72 | 129.02
Basic and diluted earnings (losses) per share (usd per share) | $ (95.53) | $ 227.72 | $ 130.02

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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 Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=125511455&loc=d3e1337-109256 + Details
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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, after giving effect to all dilutive potential common shares outstanding during the period. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 60B -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=125511455&loc=SL5780133-109256 + Details
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