Held for Sale and Discontinued Operations - EPS (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 40.8	$ 562.1	$ (3.7)	$ 604.7
Less total loss from continuing operations, net of tax	58.1	23.0	13.5	31.5
Net income from discontinued operations attributable to White Mountains’s common shareholders	(17.3)	539.1	(17.2)	573.2
Basic and diluted earnings per share numerators	$ 57.3	$ 22.7	$ 13.3	$ 31.1
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,180,400	4,297,200	3,450,800	4,477,000
Average unvested restricted common shares	(41,500)	(53,700)	(39,700)	(54,500)
Basic earnings per share denominator	3,138,900	4,243,500	3,411,100	4,422,500
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,180,400	4,297,200	3,450,800	4,477,000
Average unvested restricted common shares	(41,500)	(53,700)	(39,700)	(54,500)
Diluted earnings per share denominator	3,138,900	4,243,500	3,411,100	4,422,500
Basic and diluted earnings per share	$ 18.27	$ 5.36	$ 3.90	$ 7.03
Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 40.8	$ 562.1	$ (3.7)	$ 604.7
Less total loss from continuing operations, net of tax	58.1	23.0	13.5	31.5
Net income from discontinued operations attributable to White Mountains’s common shareholders	(17.3)	539.1	(17.2)	573.2
Allocation of income for participating unvested restricted common shares	0.2	(6.7)	0.2	(7.0)
Basic and diluted earnings per share numerators	$ (17.1)	$ 532.4	$ (17.0)	$ 566.2
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,180,400	4,297,200	3,450,800	4,477,000
Average unvested restricted common shares	(41,500)	(53,700)	(39,700)	(54,500)
Basic earnings per share denominator	3,138,900	4,243,500	3,411,100	4,422,500
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,180,400	4,297,200	3,450,800	4,477,000
Average unvested restricted common shares	(41,500)	(53,700)	(39,700)	(54,500)
Diluted earnings per share denominator	3,138,900	4,243,500	3,411,100	4,422,500
Basic and diluted earnings per share	$ (5.44)	$ 125.45	$ (4.98)	$ 128.03

The amount of net income or loss for the period per each share in instances when basic and diluted earnings per share are the same amount and reported as a single line item on the face of the financial statements. Basic earnings per share is the amount of net income or loss for the period per each share of common stock or unit outstanding during the reporting period. Diluted earnings per share includes the amount of net income or loss for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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