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Earnings Per Share - Basic and Diluted Earnings Per Share from Continuing Operations (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2024
Jun. 30, 2023
Jun. 30, 2024
Jun. 30, 2023
Basic and diluted earnings per share numerators (in millions):        
Net income (loss) attributable to White Mountains’s common shareholders $ (54.6) $ 19.6 $ 181.8 $ 199.1
Allocation of losses (earnings) to participating restricted common shares 0.7 (0.3) (2.2) (2.7)
Basic earnings (losses) per share numerators, Total (53.9) 19.3 179.6 196.4
Diluted earnings (losses) per share numerators, Total $ (53.9) $ 19.3 $ 179.6 $ 196.4
Basic earnings per share denominators (in thousands):        
Total average common shares outstanding during the period (in shares) 2,566,600 2,562,400 2,563,200 2,566,100
Average unvested restricted common shares (in shares) (34,800) (37,600) (31,700) (34,400)
Basic earnings (losses) per share denominator (in shares) 2,531,800 2,524,800 2,531,500 2,531,700
Diluted earnings per share denominator (in thousands):        
Total average common shares outstanding during the period (in shares) 2,566,600 2,562,400 2,563,200 2,566,100
Average unvested restricted common shares (in shares) (34,800) (37,600) (31,700) (34,400)
Diluted earnings (losses) per share denominator (in shares) 2,531,800 2,524,800 2,531,500 2,531,700
Basic and diluted earnings per share (in dollars):        
Distributed earnings - dividends declared and paid (in usd per share) $ 0 $ 0 $ 1.00 $ 1.00
Distributed earnings - dividends declared and paid (in usd per share) 0 0 1.00 1.00
Undistributed earnings (losses) (in usd per share) (21.24) 7.65 69.93 76.57
Undistributed earnings (losses) (in usd per share) (21.24) 7.65 69.93 76.57
Basic and diluted earnings (losses) per share (in usd per share) (21.24) 7.65 70.93 77.57
Basic and diluted earnings (losses) per share (in usd per share) $ (21.24) $ 7.65 $ 70.93 $ 77.57