Held for Sale and Discontinued Operations - EPS (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 3.5	$ 13.6	$ (44.5)	$ 42.6
Less total loss from continuing operations, net of tax	3.5	10.8	(44.6)	8.5
Net income from discontinued operations attributable to White Mountains’s common shareholders	0.0	2.8	0.1	34.1
Basic and diluted earnings per share numerators	$ 3.5	$ 10.7	$ (44.2)	$ 8.3
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,432,000	4,572,100	3,588,200	4,568,400
Average unvested restricted common shares	(41,500)	(57,200)	(38,700)	(54,900)
Basic earnings per share denominator	3,390,500	4,514,900	3,549,500	4,513,500
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,432,000	4,572,100	3,588,200	4,568,400
Average unvested restricted common shares	(41,500)	(57,200)	(38,700)	(54,900)
Diluted earnings per share denominator	3,390,500	4,514,900	3,549,500	4,513,500
Basic and diluted earnings per share	$ 1.02	$ 2.36	$ (12.44)	$ 1.84
Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net income (loss) attributable to White Mountains’s common shareholders	$ 3.5	$ 13.6	$ (44.5)	$ 42.6
Less total loss from continuing operations, net of tax	3.5	10.8	(44.6)	8.5
Net income from discontinued operations attributable to White Mountains’s common shareholders	0.0	2.8	0.1	34.1
Allocation of income for participating unvested restricted common shares	0.0	0.0	0.0	(0.4)
Basic and diluted earnings per share numerators	$ 0.0	$ 2.8	$ 0.1	$ 33.7
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	3,432,000	4,572,100	3,588,200	4,568,400
Average unvested restricted common shares	(41,500)	(57,200)	(38,700)	(54,900)
Basic earnings per share denominator	3,390,500	4,514,900	3,549,500	4,513,500
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	3,432,000	4,572,100	3,588,200	4,568,400
Average unvested restricted common shares	(41,500)	(57,200)	(38,700)	(54,900)
Diluted earnings per share denominator	3,390,500	4,514,900	3,549,500	4,513,500
Basic and diluted earnings per share	$ 0.00	$ 0.61	$ 0.03	$ 7.47