Earnings Per Share - Undistributed Net Earnings From Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Earnings Per Share [Abstract]
Net income (losses) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ (172.9)	$ 135.7	$ (143.7)	$ 71.6
Dividends declared net of restricted common share amounts	0.0	0.0	(3.0)	(3.1)
Total undistributed net earnings (losses), net of restricted common share amounts	$ (172.9)	$ 135.7	$ (146.7)	$ 68.5

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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