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Earnings Per Share - Schedule of Earnings Per Share from Continuing Operations (Details) - USD ($)
$ / shares in Units, $ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Basic and diluted earnings per share numerators (in millions):      
Net income (loss) attributable to White Mountains’s common shareholders $ 1,106.4 $ 230.4 $ 509.2
Adjustment for redeemable noncontrolling interests (2.8) 0.0 0.0
Allocation of (earnings) losses to participating restricted common shares (13.5) (3.0) (7.2)
Net income (loss) attributable to White Mountains’s common shareholders 1,090.1 227.4 502.0
Diluted earnings (losses) per share numerators $ 1,090.1 $ 227.4 $ 502.0
Basic earnings per share denominators (in thousands):      
Total average common shares outstanding during the period (in shares) 2,565,500 2,565,700 2,563,300
Average unvested restricted common shares (in shares) (31,200) (33,500) (36,000.0)
Basic earnings (loss) per share denominator (in shares) 2,534,300 2,532,200 2,527,300
Diluted earnings per share denominator (in thousands):      
Total average common shares outstanding during the period (in shares) 2,565,500 2,565,700 2,563,300
Average unvested restricted common shares (in shares) (31,200) (33,500) (36,000.0)
Diluted earnings (loss) per share denominator (in shares) 2,534,300 2,532,200 2,527,300
Basic and diluted earnings per share (in dollars):      
Distributed earnings - dividends declared and paid (in usd per share) $ 1.00 $ 1.00 $ 1.00
Distributed earnings - dividends declared and paid (in usd per share) 1.00 1.00 1.00
Undistributed earnings (losses) (in usd per share) 429.14 88.79 197.60
Undistributed earnings (losses) (in usd per share) 429.14 88.79 197.60
Basic and diluted earnings (losses) per share (in usd per share) 430.14 89.79 198.60
Basic and diluted earnings (losses) per share (in usd per share) $ 430.14 $ 89.79 $ 198.60