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Held for Sale and Discontinued Operations - Earnings Per Share for Discontinued 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)	$ 33.4	$ (75.3)
Less: total income (loss) from continuing operations, net of tax	33.4	(94.0)
Net gain from discontinued operations attributable to White Mountains’s common shareholders	0.0	18.7
Allocation of 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 (losses) earnings per share (in usd per share)	$ 11.10	$ (30.33)
Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net income (loss)	$ 33.4	$ (75.3)
Less: total income (loss) from continuing operations, net of tax	33.4	(94.0)
Net gain from discontinued operations attributable to White Mountains’s common shareholders	0.0	18.7
Allocation of earnings to participating restricted common shares	0.0	(0.2)
Basic and diluted earnings (losses) per share numerators	$ 0.0	$ 18.5
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 (losses) earnings per share (in usd per share)	$ 0	$ 6.03

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