Earnings Per Share - Undistributed Net Earnings (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 114.8	$ 20.2	$ (13.2)	$ 300.6
Dividends declared net of restricted common share amounts	0.0	0.0	(3.1)	(3.2)
Total undistributed net earnings (losses), net of restricted common share amounts	$ 114.8	$ 20.2	$ (16.3)	$ 297.4

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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