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Earnings Per Share - Schedule of Undistributed Net Earnings (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2025 | Dec. 31, 2024 | Dec. 31, 2023
Earnings Per Share [Abstract] |  |  |
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 | (2.5) | (2.5) | (2.5)
Total undistributed net earnings (losses), net of restricted common share amounts | $ 1,087.6 | $ 224.9 | $ 499.5

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