Earnings Per Share - Schedule of Undistributed Net Earnings (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2025	Mar. 31, 2024
Earnings Per Share [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders, net of restricted common share amounts	$ 33.6	$ 233.7
Dividends declared, net of restricted common share amounts	(2.5)	(2.5)
Total undistributed net earnings (losses), net of restricted common share amounts	$ 31.1	$ 231.2

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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