Earnings Per Share - Undistributed Net Earnings (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 57.3	$ 22.7	$ 13.3	$ 31.1
Dividends declared net of restricted common share amounts	0.0	0.0	(3.7)	(4.5)
Total undistributed net losses, net of restricted common share amounts	$ 57.3	$ 22.7	$ 9.6	$ 26.6

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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