Earnings Per Share - Basic and Diluted Earnings Per Share from Continuing Operations (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$ 232.9	$ 48.7	$ 219.5	$ 353.6
Less: total (loss) income from discontinued operations, net of tax	(0.7)	0.9	(0.8)	1.6
Net income from continuing operations attributable to White Mountains’s common shareholders	233.6	47.8	220.3	352.0
Allocation of income for participating restricted common shares	(3.2)	(0.7)	(2.9)	(4.4)
Basic and diluted earnings per share numerators	$ 230.4	$ 47.1	$ 217.4	$ 347.6
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period (in shares)	3,101,800	3,185,400	3,129,000.0	3,180,400
Average unvested restricted common shares (in shares)	(43,100)	(43,400)	(40,000.0)	(39,600)
Basic earnings (losses) per share denominator (in shares)	3,058,700	3,142,000.0	3,089,000.0	3,140,800
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period (in shares)	3,101,800	3,185,400	3,129,000.0	3,180,400
Average unvested restricted common shares (in shares)	(43,100)	(43,400)	(40,000.0)	(39,600)
Diluted earnings (losses) per share denominator (in shares)	3,058,700	3,142,000.0	3,089,000.0	3,140,800
Basic and diluted earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid (in USD per share)	$ 0	$ 0	$ 1.00	$ 1.00
Undistributed earnings (losses) (in USD per share)	75.32	15.01	69.40	109.67
Basic and diluted earnings (losses) per share (in USD per share)	$ 75.32	$ 15.01	$ 70.40	$ 110.67