Earnings Per Share - Basic and Diluted Earnings Per Share from Continuing Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 33.4	$ (75.3)
Less: total income (loss) from discontinued operations, net of tax	0.0	(18.7)
Less: total income (loss) from continuing operations, net of tax	33.4	(94.0)
Allocation of losses (earnings) to participating restricted common shares	(0.3)	0.9
Basic and diluted earnings (losses) per share numerators	$ 33.1	$ (93.1)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	3,006,200	3,100,200
Average unvested restricted common shares (in shares)	(29,600)	(32,400)
Basic earnings (losses) per share denominator (in shares)	2,976,600	3,067,800
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	3,006,200	3,100,200
Average unvested restricted common shares (in shares)	(29,600)	(32,400)
Diluted earnings (losses) per share denominator (in shares)	2,976,600	3,067,800
Basic and diluted earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid (in usd per share)	$ 1.00	$ 1.00
Undistributed (losses) earnings (in usd per share)	10.10	(31.33)
Basic and diluted (losses) earnings per share (in usd per share)	$ 11.10	$ (30.33)