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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, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Basic and diluted earnings per share numerators (in millions): |  |  |
Net income (loss) attributable to White Mountains’s common shareholders | $ 792.8 | $ (275.4) | $ 708.7
Less: total income (loss) from discontinued operations, net of tax | 903.2 | (3.9) | (11.8)
Less: net (income) loss from discontinued operations attributable to non-controlling interests | (0.7) | 1.0 | 0.2
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders | (109.7) | (272.5) | 720.3
Allocation of (earnings) losses to participating restricted common shares | 1.3 | 3.2 | (9.5)
Basic and diluted earnings (losses) per share numerators | $ (108.4) | $ (269.3) | $ 710.8
Basic earnings per share denominators (in thousands): |  |  |
Total average common shares outstanding during the period (in shares) | 2,862,400 | 3,079,000 | 3,122,200
Average unvested restricted common shares (in shares) | (36,200) | (36,500) | (40,800)
Basic earnings (losses) per share denominator (in shares) | 2,826,200 | 3,042,500 | 3,081,400
Diluted earnings per share denominator (in thousands): |  |  |
Average outstanding dilutive options to acquire common shares (in shares) | 2,862,400 | 3,079,000 | 3,122,200
Average unvested restricted common shares (in shares) | (36,200) | (36,500) | (40,800)
Diluted earnings (loss) per share denominator (in shares) | 2,826,200 | 3,042,500 | 3,081,400
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) | (39.34) | (89.52) | 229.69
Total consolidated operations (usd per share) | (38.34) | (88.52) | 230.69
Total consolidated operations (usd per share) | $ (38.34) | $ (88.52) | $ 230.69

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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 https://asc.fasb.org/extlink&oid=126958026&loc=SL5780133-109256 + Details
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- Definition The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=124432515&loc=d3e3550-109257 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 16 -URI https://asc.fasb.org/extlink&oid=126958026&loc=d3e1505-109256 + Details
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- Definition This element represents the weighted average total number of shares issued throughout the period including the first (beginning balance outstanding) and last (ending balance outstanding) day of the period before considering any reductions (for instance, shares held in treasury) to arrive at the weighted average number of shares outstanding. Weighted average relates to the portion of time within a reporting period that common shares have been issued and outstanding to the total time in that period. Such concept is used in determining the weighted average number of shares outstanding for purposes of calculating earnings per share (basic). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 10 -URI https://asc.fasb.org/extlink&oid=126958026&loc=d3e1448-109256 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 13 -URI https://asc.fasb.org/extlink&oid=126958026&loc=d3e2646-109256 + Details
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- Definition Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (a) -URI https://asc.fasb.org/extlink&oid=124432515&loc=d3e3550-109257 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 10 -URI https://asc.fasb.org/extlink&oid=126958026&loc=d3e1448-109256 + 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 13 -URI https://asc.fasb.org/extlink&oid=126958026&loc=d3e2646-109256 + Details
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- Definition Represents the portion of income (loss) allocated to participating unvested restricted common shares. + References No definition available. + Details
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- Definition Net Income (Loss) Attributable to Parent, Basic and Diluted Earnings Per Share Numerators + References No definition available. + Details
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