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Earnings Per Share (Schedule of Undistributed Net Earnings) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 408.4	$ (122.6)	$ 49.0
Dividends declared, net of restricted common share amounts	(3.2)	(3.7)	(4.5)
Total undistributed net earnings (losses), net of restricted common share amounts	$ 405.2	$ (126.3)	$ 44.5

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The earnings that is allocated to common stock and participating securities to the extent that each security may share in earnings as if all of the earnings for the period had been distributed.

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Equity impact of common stock cash dividends declared by the entity during the period, net of restricted share amounts.

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Represents Net income from continuing operations attributable to White Mountains common shareholders, net of allocation to restricted shares and dividends paid on restricted shares.

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