Earnings Per Share - Schedule of Undistributed Net Earnings (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2026	Sep. 30, 2025	Sep. 30, 2026	Sep. 30, 2025
Earnings Per Share [Abstract]
Net earnings (losses) available to White Mountains’s common shareholders, net of restricted common share amounts	$ (41.8)	$ 112.3	$ 121.7	$ 267.3
Dividends declared, net of restricted common share amounts	0.0	0.0	(2.4)	(2.5)
Total undistributed net earnings (losses), net of restricted common share amounts	$ (41.8)	$ 112.3	$ 119.3	$ 264.8

Amount, after deduction of tax, noncontrolling interests, dividends on preferred stock and participating securities; of income (loss) available to common shareholders.

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+ Details