Held for Sale and Discontinued Operations - Earnings Per Share for Discontinued Operations (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 230.4	$ 509.2	$ 792.8
Less: net income (loss) from continuing operations	$ 284.1	$ 580.9	(190.8)
Less: net (income) loss from continuing operations attributable to noncontrolling interest			81.1
Total income (loss) from discontinued operations attributable to White Mountains’s common shareholders			902.5
Allocation of (earnings) losses to participating restricted common shares			(11.4)
Basic earnings (loss) per share numerators			891.1
Diluted earnings (loss) per share numerators			$ 891.1
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	2,565,700	2,563,300	2,862,400
Average unvested restricted common shares (in shares)	(33,500)	(36,000.0)	(36,200)
Basic earnings (losses) per share denominator (in shares)	2,532,200	2,527,300	2,826,200
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	2,565,700	2,563,300	2,862,400
Average unvested restricted common shares (in shares)	(33,500)	(36,000.0)	(36,200)
Diluted earnings (loss) per share denominator (in shares)	2,532,200	2,527,300	2,826,200
Basic (loss) earnings per share (in dollars) - discontinued operations (in dollars per share)	$ 0	$ 0	$ 315.30
Diluted (loss) earnings per share (in dollars) - discontinued operations (in dollars per share)	$ 0	$ 0	$ 315.30