Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Basic and diluted earnings per share numerators (in millions):
Net (loss) income attributable to White Mountains’s common shareholders	$ (128.8)	$ 284.4
Less: total income (loss) from discontinued operations	0.9	0.7
Net (loss) income from continuing operations attributable to White Mountains’s common shareholders	(129.7)	283.7
Basic and diluted (losses) earnings per share numerators	$ (128.4)	$ 280.8
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,178,000.0	3,172,200
Average unvested restricted common shares	(33,800)	(32,500)
Basic (losses) earnings per share denominator	3,144,200	3,139,700
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,178,000.0	3,172,200
Average unvested restricted common shares	(33,800)	(32,500)
Diluted (losses) earnings per share denominator	3,144,200	3,139,700
Basic earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid	$ 1.00	$ 1.00
Undistributed (losses) earnings	(41.82)	88.42
Basic and diluted (losses) earnings per share	$ (40.82)	$ 89.42