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Earnings Per Share - Undistributed Net Earnings From Continuing Operations (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2024 | Sep. 30, 2023
Earnings Per Share [Abstract] |  |  |  |
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts | $ 176.5 | $ 23.3 | $ 356.2 | $ 219.6
Dividends declared, net of restricted common share amounts | 0.0 | 0.0 | (2.5) | (2.5)
Total undistributed net earnings (losses), net of restricted common share amounts | $ 176.5 | $ 23.3 | $ 353.7 | $ 217.1

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