Earnings Per Share - Schedule of Earnings Per Share from Continuing Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 113.8	$ 179.0	$ 270.6	$ 360.8
Allocation of losses (earnings) to participating restricted common shares	(1.5)	(2.5)	(3.3)	(4.6)
Basic earnings (losses) per share numerators, Total	112.3	176.5	267.3	356.2
Diluted earnings (losses) per share numerators, Total	$ 112.3	$ 176.5	$ 267.3	$ 356.2
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	2,575,200	2,568,100	2,572,200	2,564,800
Average unvested restricted common shares (in shares)	(32,900)	(35,400)	(30,700)	(32,900)
Basic earnings (losses) per share denominator (in shares)	2,542,300	2,532,700	2,541,500	2,531,900
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	2,575,200	2,568,100	2,572,200	2,564,800
Average unvested restricted common shares (in shares)	(32,900)	(35,400)	(30,700)	(32,900)
Diluted earnings (losses) per share denominator (in shares)	2,542,300	2,532,700	2,541,500	2,531,900
Basic and diluted earnings per share (in dollars):
Distributed earnings - dividends declared and paid (in dollars per share)	$ 0	$ 0	$ 1.00	$ 1.00
Distributed earnings - dividends declared and paid (in dollars per share)	0	0	1.00	1.00
Undistributed earnings (losses) (in dollars per share)	44.18	69.68	104.18	139.66
Undistributed earnings (losses) (in dollars per share)	44.18	69.68	104.18	139.66
Basic and diluted earnings (losses) per share (in dollars per share)	44.18	69.68	105.18	140.66
Basic and diluted earnings (losses) per share (in dollars per share)	$ 44.18	$ 69.68	$ 105.18	$ 140.66