Earnings Per Share - Undistributed Net Earnings From Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net (losses) earnings attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ (366.9)	$ 230.4	$ (323.1)	$ 217.4
Dividends declared net of restricted common share amounts	0.0	0.0	(3.1)	(3.1)
Total undistributed net (losses) earnings, net of restricted common share amounts	$ (366.9)	$ 230.4	$ (326.2)	$ 214.3

Amount of paid and unpaid cash, stock, and paid-in-kind (PIK) dividends declared, for example, but not limited to, common and preferred stock.

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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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Net Income (Loss) Attributable to Parent, Net of Restricted Common Share Amounts

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