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Earnings Per Share - Schedule of Undistributed Net Earnings (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Earnings Per Share [Abstract]      
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 502.0 $ (108.4) $ (269.3)
Dividends declared, net of restricted common share amounts (2.5) (3.0) (3.1)
Total undistributed net earnings (losses), net of restricted common share amounts $ 499.5 $ (111.4) $ (272.4)