Held for Sale and Discontinued Operations - EPS (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 continuing operations, net of tax	116.4	20.5	(13.3)	304.2
Net (loss) income from discontinued operations attributable to White Mountains’s common shareholders	(1.0)	0.0	(0.1)	0.7
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 per share denominator	3,064,400	3,140,700	3,104,300	3,140,200
Earnings Per Share, Basic and Diluted	$ 37.14	$ 6.44	$ (4.28)	$ 95.93
Discontinued Operations
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 continuing operations, net of tax	116.4	20.5	(13.3)	304.2
Net (loss) income from discontinued operations attributable to White Mountains’s common shareholders	(1.0)	0.0	(0.1)	0.7
Allocation of income for participating unvested restricted common shares	0.0	0.0	0.0	0.0
Basic and diluted earnings (losses) per share numerators	$ (1.0)	$ 0.0	$ (0.1)	$ 0.7
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 per share denominator	3,064,400	3,140,700	3,104,300	3,140,200
Earnings Per Share, Basic and Diluted	$ (0.32)	$ 0	$ (0.03)	$ 0.22