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Earnings Per Share - Undistributed Net Earnings From Continuing Operations (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2024 | Mar. 31, 2023
Earnings Per Share [Abstract] |  |
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts | $ 233.7 | $ 177.3
Dividends declared, net of restricted common share amounts | (2.5) | (2.5)
Total undistributed net earnings (losses), net of restricted common share amounts | $ 231.2 | $ 174.8

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