v3.26.1
Earnings Per Share - Schedule of Earnings Per Share from Continuing Operations (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Basic and diluted earnings per share numerators (in millions):    
Net income (loss) attributable to White Mountains’s common shareholders $ (27.2) $ 33.9
Adjustment for redeemable noncontrolling interests (3.9) 0.0
Allocation of losses (earnings) to participating restricted common shares 0.3 (0.3)
Basic earnings (losses) per share numerators, Total (30.8) 33.6
Diluted earnings (losses) per share numerators, Total $ (30.8) $ 33.6
Basic earnings per share denominators (in thousands):    
Total average common shares outstanding during the period (in shares) 2,472,800 2,567,000
Average unvested restricted common shares (in shares) (25,500) (26,100)
Basic earnings (losses) per share denominator (in shares) 2,447,300 2,540,900
Diluted earnings per share denominator (in thousands):    
Total average common shares outstanding during the period (in shares) 2,472,800 2,567,000
Average unvested restricted common shares (in shares) (25,500) (26,100)
Diluted earnings (losses) per share denominator (in shares) 2,447,300 2,540,900
Basic and diluted earnings per share (in dollars):    
Distributed earnings - dividends declared and paid (in dollars per share) $ 1.00 $ 1.00
Distributed earnings - dividends declared and paid (in dollars per share) 1.00 1.00
Undistributed earnings (losses) (in dollars per share) (13.59) 12.19
Undistributed earnings (losses) (in dollars per share) (13.59) 12.19
Basic earnings (losses) per share (in dollars per share) (12.59) 13.19
Diluted earnings (losses) per share (in dollars per share) $ (12.59) $ 13.19