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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, 2021	Mar. 31, 2020
Basic and diluted loss per share numerators (in millions):
Net loss attributable to White Mountains’s common shareholders	$ (75.3)	$ (128.8)
Less: total loss from continuing operations, net of tax	(94.0)	(129.7)
Net income from discontinued operations attributable to White Mountains’s common shareholders	18.7	0.9
Allocation of earnings to participating restricted common shares	0.9	1.3
Basic and diluted losses per share numerators	$ (93.1)	$ (128.4)
Basic loss per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	3,100,200	3,178,000.0
Average unvested restricted common shares (in shares)	(32,400)	(33,800)
Basic earnings (losses) per share denominator (in shares)	3,067,800	3,144,200
Diluted loss per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	3,100,200	3,178,000.0
Average unvested restricted common shares (in shares)	(32,400)	(33,800)
Diluted earnings (losses) per share denominator (in shares)	3,067,800	3,144,200
Basic and diluted earnings (losses) per share (in USD per share)	$ (30.33)	$ (40.82)
Discontinued Operations
Basic and diluted loss per share numerators (in millions):
Net loss attributable to White Mountains’s common shareholders	$ (75.3)	$ (128.8)
Less: total loss from continuing operations, net of tax	(94.0)	(129.7)
Net income from discontinued operations attributable to White Mountains’s common shareholders	18.7	0.9
Allocation of earnings to participating restricted common shares	(0.2)	0.0
Basic and diluted losses per share numerators	$ 18.5	$ 0.9
Basic loss per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	3,100,200	3,178,000.0
Average unvested restricted common shares (in shares)	(32,400)	(33,800)
Basic earnings (losses) per share denominator (in shares)	3,067,800	3,144,200
Diluted loss per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	3,100,200	3,178,000.0
Average unvested restricted common shares (in shares)	(32,400)	(33,800)
Diluted earnings (losses) per share denominator (in shares)	3,067,800	3,144,200
Basic and diluted earnings (losses) per share (in USD per share)	$ 6.03	$ 0.28

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