Earnings Per Share - Schedule of Earnings Per Share from Continuing Operations (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 792.8	$ (275.4)	$ 708.7
Less: total income (loss) from discontinued operations, net of tax	903.2	(3.9)	(11.8)
Less: net (income) loss from discontinued operations attributable to non-controlling interests	(0.7)	1.0	0.2
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	(109.7)	(272.5)	720.3
Allocation of (earnings) losses to participating restricted common shares	1.3	3.2	(9.5)
Basic and diluted earnings (losses) per share numerators	$ (108.4)	$ (269.3)	$ 710.8
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	2,862,400	3,079,000	3,122,200
Average unvested restricted common shares (in shares)	(36,200)	(36,500)	(40,800)
Basic earnings (losses) per share denominator (in shares)	2,826,200	3,042,500	3,081,400
Diluted earnings per share denominator (in thousands):
Average outstanding dilutive options to acquire common shares (in shares)	2,862,400	3,079,000	3,122,200
Average unvested restricted common shares (in shares)	(36,200)	(36,500)	(40,800)
Diluted earnings (loss) per share denominator (in shares)	2,826,200	3,042,500	3,081,400
Diluted earnings per share (in dollars) - continuing operations:
Dividends declared and paid (usd per share)	$ 1.00	$ 1.00	$ 1.00
Undistributed earnings (losses) (usd per share)	(39.34)	(89.52)	229.69
Total consolidated operations (usd per share)	(38.34)	(88.52)	230.69
Total consolidated operations (usd per share)	$ (38.34)	$ (88.52)	$ 230.69