Held for Sale and Discontinued Operations - Earnings Per Share for Discontinued Operations (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 509.2	$ 792.8	$ (275.4)
Less: net income (loss) from continuing operations	$ 580.9	(190.8)	(318.0)
Less: net (income) loss from continuing operations attributable to noncontrolling interest		81.1	45.5
Total gain (loss) from discontinued operations attributable to White Mountains’s common shareholders		902.5	(2.9)
Allocation of earnings to participating restricted common shares		(11.4)	0.0
Basic earnings per share numerators		891.1	(2.9)
Diluted earnings per share numerators		$ 891.1	$ (2.9)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	2,563,300	2,862,400	3,079,000
Average unvested restricted common shares (in shares)	(36,000.0)	(36,200)	(36,500)
Basic earnings (losses) per share denominator (in shares)	2,527,300	2,826,200	3,042,500
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	2,563,300	2,862,400	3,079,000
Average unvested restricted common shares (in shares)	(36,000.0)	(36,200)	(36,500)
Diluted earnings (loss) per share denominator (in shares)	2,527,300	2,826,200	3,042,500
Basic earnings (losses) per share (in dollars) - discontinued operations (in dollars per share)	$ 0	$ 315.30	$ (0.94)
Diluted earnings (losses) per share (in dollars) - discontinued operations (in dollars per share)	$ 0	$ 315.30	$ (0.94)