Earnings Per Share - Undistributed Net Earnings From Continuing Operations (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2022	Mar. 31, 2021
Earnings Per Share [Abstract]
Net income (losses) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 33.1	$ (93.1)
Dividends declared net of restricted common share amounts	(3.0)	(3.1)
Total undistributed net earnings (losses), net of restricted common share amounts	$ 30.1	$ (96.2)

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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- Definition

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.

+ References

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- Definition

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

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