Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 48.7	$ 40.8	$ 353.6	$ (3.7)
Less: total income (loss) from discontinued operations	0.9	(17.3)	1.6	(17.2)
Net income from continuing operations attributable to White Mountains’s common shareholders	47.8	58.1	352.0	13.5
Allocation of earnings to participating restricted common shares	(0.7)	(0.8)	(4.4)	(0.2)
Basic and diluted earnings per share numerators	$ 47.1	$ 57.3	$ 347.6	$ 13.3
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,185,400	3,180,400	3,180,400	3,450,800
Average unvested restricted common shares	(43,400)	(41,500)	(39,600)	(39,700)
Basic earnings per share denominator	3,142,000.0	3,138,900	3,140,800	3,411,100
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,185,400	3,180,400	3,180,400	3,450,800
Average unvested restricted common shares	(43,400)	(41,500)	(39,600)	(39,700)
Diluted earnings per share denominator	3,142,000.0	3,138,900	3,140,800	3,411,100
Basic earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid	$ 0	$ 0	$ 1.00	$ 1.00
Undistributed earnings	15.01	18.27	109.67	2.90
Basic and diluted earnings per share	$ 15.01	$ 18.27	$ 110.67	$ 3.90