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Held for Sale and Discontinued Operations - EPS (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Basic and diluted earnings per share numerators (in millions):
Net (loss) income attributable to White Mountains’s common shareholders	$ (128.8)	$ 284.4
Less: total (loss) income from continuing operations, net of tax	(129.7)	283.7
Net income from discontinued operations attributable to White Mountains’s common shareholders	0.9	0.7
Allocation of income for participating unvested restricted common shares	1.3	(2.9)
Basic and diluted (losses) earnings per share numerators	$ (128.4)	$ 280.8
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,178,000.0	3,172,200
Average unvested restricted common shares	(33,800)	(32,500)
Basic (losses) earnings per share denominator	3,144,200	3,139,700
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,178,000.0	3,172,200
Average unvested restricted common shares	(33,800)	(32,500)
Diluted earnings per share denominator	3,144,200	3,139,700
Earnings Per Share, Basic and Diluted	$ (40.54)	$ 89.64
Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net (loss) income attributable to White Mountains’s common shareholders	$ (128.8)	$ 284.4
Less: total (loss) income from continuing operations, net of tax	(129.7)	283.7
Net income from discontinued operations attributable to White Mountains’s common shareholders	0.9	0.7
Allocation of income for participating unvested restricted common shares	0.0	0.0
Basic and diluted (losses) earnings per share numerators	$ 0.9	$ 0.7
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,178,000.0	3,172,200
Average unvested restricted common shares	(33,800)	(32,500)
Basic (losses) earnings per share denominator	3,144,200	3,139,700
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,178,000.0	3,172,200
Average unvested restricted common shares	(33,800)	(32,500)
Diluted earnings per share denominator	3,144,200	3,139,700
Earnings Per Share, Basic and Diluted	$ 0.28	$ 0.22

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