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Earnings Per Share (Tables) | 12 Months Ended
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Dec. 31, 2025
Earnings Per Share [Abstract] |
Schedule of earnings per share from continuing operations | The following table presents the Company’s computation of earnings per share for the years ended December 31, 2025, 2024 and 2023.
|  | Year Ended December 31,
|  | 2025 |  | 2024 |  | 2023
Basic and diluted earnings per share numerators (in millions): |  |  |  |  |  |
Net income (loss) attributable to White Mountains’s common shareholders |  | $ | 1,106.4 |  |  | $ | 230.4 |  |  | $ | 509.2 |
Adjustment for redeemable noncontrolling interests (1) |  | (2.8) |  |  | — |  |  | — |
Allocation of (earnings) losses to participating restricted common shares (2) |  | (13.5) |  |  | (3.0) |  |  | (7.2) |
Basic and diluted earnings (losses) per share numerators |  | $ | 1,090.1 |  |  | $ | 227.4 |  |  | $ | 502.0 |
Basic earnings per share denominators (in thousands): |  |  |  |  |  |
Total average common shares outstanding during the period |  | 2,565.5 |  |  | 2,565.7 |  |  | 2,563.3 |
Average unvested restricted common shares (3) |  | (31.2) |  |  | (33.5) |  |  | (36.0) |
Basic earnings (losses) per share denominator |  | 2,534.3 |  |  | 2,532.2 |  |  | 2,527.3 |
Diluted earnings per share denominator (in thousands): |  |  |  |  |  |
Total average common shares outstanding during the period |  | 2,565.5 |  |  | 2,565.7 |  |  | 2,563.3 |
Average unvested restricted common shares (3) |  | (31.2) |  |  | (33.5) |  |  | (36.0) |
Diluted earnings (losses) per share denominator |  | 2,534.3 |  |  | 2,532.2 |  |  | 2,527.3 |
Basic and diluted earnings per share (in dollars): |  |  |  |  |  |
Distributed earnings - dividends declared and paid |  | $ | 1.00 |  |  | $ | 1.00 |  |  | $ | 1.00 |
Undistributed earnings (losses) |  | 429.14 |  |  | 88.79 |  |  | 197.60 |
Basic and diluted earnings (losses) per share |  | $ | 430.14 |  |  | $ | 89.79 |  |  | $ | 198.60 |
(1) White Mountains recognizes an adjustment to the carrying value of the redeemable noncontrolling interests to the extent that the carrying value is below the redemption value. When calculating basic and diluted earnings per share, the adjustment for redeemable noncontrolling interests is treated as being akin to a dividend and presented as an adjustment to net income (loss) attributable to White Mountains’s common shareholders. (2) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities. (3) Restricted shares outstanding vest upon a stated date. See Note 12 — “Employee Share-Based Incentive Compensation Plans.”
Schedule of undistributed earnings | The following table presents the undistributed net earnings (losses) for the years ended December 31, 2025, 2024 and 2023.
|  |  | Year Ended December 31,
Millions |  |  | 2025 |  | 2024 |  | 2023
Undistributed net earnings: |  |  |  |  |  |  |
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts |  |  | $ | 1,090.1 |  |  | $ | 227.4 |  |  | $ | 502.0 |
Dividends declared, net of restricted common share amounts (1) |  |  | (2.5) |  |  | (2.5) |  |  | (2.5) |
Total undistributed net earnings (losses), net of restricted common share amounts |  |  | $ | 1,087.6 |  |  | $ | 224.9 |  |  | $ | 499.5 |
(1) Restricted shares issued by White Mountains receive dividends, and therefore, are considered participating securities.

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- Definition Tabular disclosure of an entity's basic and diluted earnings per share calculations, including a reconciliation of numerators and denominators of the basic and diluted per-share computations for income from continuing operations. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -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 + Details
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- Definition Schedule of Undistributed Earnings + References No definition available. + Details
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