Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 115.4	$ 20.5	$ (13.4)	$ 304.9
Less: total income (loss) from discontinued operations	(1.0)	0.0	(0.1)	0.7
Net income (loss) from continuing operations attributable to White Mountains’s common shareholders	116.4	20.5	(13.3)	304.2
Allocation of earnings to participating restricted common shares	(1.6)	(0.3)	0.1	(3.6)
Basic and diluted earnings (losses) per share numerators	$ 114.8	$ 20.2	$ (13.2)	$ 300.6
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,107,500	3,183,500	3,142,800	3,177,800
Average unvested restricted common shares	(43,100)	(42,800)	(38,500)	(37,600)
Basic earnings (losses) per share denominator	3,064,400	3,140,700	3,104,300	3,140,200
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,107,500	3,183,500	3,142,800	3,177,800
Average unvested restricted common shares	(43,100)	(42,800)	(38,500)	(37,600)
Diluted earnings (losses) per share denominator	3,064,400	3,140,700	3,104,300	3,140,200
Basic earnings per share (in dollars) - continuing operations:
Distributed earnings - dividends declared and paid	$ 0	$ 0	$ 1.00	$ 1.00
Undistributed earnings (losses)	37.46	6.44	(5.25)	94.71
Basic and diluted earnings (losses) per share	$ 37.46	$ 6.44	$ (4.25)	$ 95.71