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Earnings Per Share - Schedule of Undistributed Net Earnings (Details) - USD ($)
$ in Millions
 12 Months Ended
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