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Earnings Per Share - Basic and Diluted Earnings Per Share from Continuing Operations (Details) - USD ($) $ / shares in Units, $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2023 | Jun. 30, 2022 | Jun. 30, 2023 | Jun. 30, 2022
Basic and diluted earnings per share numerators (in millions): |  |  |  |
Net income (loss) attributable to White Mountains’s common shareholders | $ 19.6 | $ (169.2) | $ 199.1 | $ (135.8)
Less: total income (loss) from discontinued operations, net of tax | 0.0 | 6.4 | 0.0 | 10.1
Net (income) loss from discontinued operations attributable to noncontrolling interests | 0.0 | (0.5) | 0.0 | (0.6)
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders | 19.6 | (175.1) | 199.1 | (145.3)
Allocation of losses (earnings) to participating restricted common shares | (0.3) | 2.2 | (2.7) | 1.6
Basic and diluted earnings (losses) per share numerators | $ 19.3 | $ (172.9) | $ 196.4 | $ (143.7)
Basic earnings per share denominators (in thousands): |  |  |  |
Total average common shares outstanding during the period | 2,562,400 | 2,979,000 | 2,566,100 | 2,992,500
Average unvested restricted common shares | (37,600) | (38,300) | (34,400) | (33,900)
Basic earnings (losses) per share denominator (in shares) | 2,524,800 | 2,940,700 | 2,531,700 | 2,958,600
Diluted earnings per share denominator (in thousands): |  |  |  |
Total average common shares outstanding during the period | 2,562,400 | 2,979,000 | 2,566,100 | 2,992,500
Average unvested restricted common shares | (37,600) | (38,300) | (34,400) | (33,900)
Diluted earnings (losses) per share denominator | 2,524,800 | 2,940,700 | 2,531,700 | 2,958,600
Basic and diluted earnings per share (in dollars) - continuing operations: |  |  |  |
Distributed earnings - dividends declared and paid (in usd per share) | $ 0 | $ 0 | $ 1.00 | $ 1.00
Undistributed earnings (losses) | 7.65 | (58.78) | 76.57 | (49.57)
Basic and diluted earnings (losses) per share (in dollars per share) | 7.65 | (58.78) | 77.57 | (48.57)
Basic and diluted earnings (losses) per share (in dollars per share) | $ 7.65 | $ (58.78) | $ 77.57 | $ (48.57)

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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 -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 13 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482689/260-10-45-13 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 10 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482689/260-10-45-10 + Details
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Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + 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 -Topic 260 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482662/260-10-50-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 260 -SubTopic 10 -Name Accounting Standards Codification -Section 45 -Paragraph 10 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482689/260-10-45-10 + 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 -Name Accounting Standards Codification -Topic 260 -SubTopic 10 -Section 45 -Paragraph 13 -Publisher FASB -URI https://asc.fasb.org//1943274/2147482689/260-10-45-13 + Details
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Period Type: | duration

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- Definition Earnings Per Share, Basic and Diluted, Numerator + References No definition available. + Details
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Namespace Prefix: | wtm_
Data Type: | xbrli:stringItemType
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Period Type: | duration

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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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Balance Type: | credit
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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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Namespace Prefix: | wtm_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration