v3.25.2
Earnings Per Share - Schedule of Undistributed Net Earnings (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2025
Jun. 30, 2024
Jun. 30, 2025
Jun. 30, 2024
Earnings Per Share [Abstract]        
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts $ 121.3 $ (53.9) $ 155.0 $ 179.6
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 $ 121.3 $ (53.9) $ 152.5 $ 177.1