Discontinued Operations Earnings Per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Basic and diluted earnings per share numerators (in millions):
Income (Loss) Allocated to Participating Unvested Restricted Common Shares	$ (0.6)	$ (0.9)	$ (3.0)	$ (2.9)
Net income attributable to White Mountains' common shareholders, net of restricted common share amounts	51.5	57.2	242.7	203.9
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	6,091,500	6,176,600	6,140,900	6,208,400
Average unvested restricted common shares	(81,300)	(95,500)	(77,700)	(90,000)
Basic earnings per share denominator	6,010,200	6,081,100	6,063,200	6,118,400
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	6,091,500	6,176,600	6,140,900	6,208,400
Average unvested restricted common shares	(81,300)	(95,500)	(77,700)	(90,000)
Average outstanding dilutive options to acquire common shares	0	0	0	0
Diluted earnings per share denominator	6,010,200	6,081,100	6,063,200	6,118,400
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	125,000	125,000	125,000	125,000
Segment, Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net income from discontinued operations	6.7	0.4	8.8	4.8
Income (Loss) Allocated to Participating Unvested Restricted Common Shares	(0.1)	0	(0.1)	0
Net income attributable to White Mountains' common shareholders, net of restricted common share amounts	$ 6.6	$ 0.4	$ 8.7	$ 4.8
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period	6,091,500	6,176,600	6,140,900	6,208,400
Average unvested restricted common shares	(81,300)	(95,500)	(77,700)	(90,000)
Basic earnings per share denominator	6,010,200	6,081,100	6,063,200	6,118,400
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period	6,091,500	6,176,600	6,140,900	6,208,400
Average unvested restricted common shares	(81,300)	(95,500)	(77,700)	(90,000)
Average outstanding dilutive options to acquire common shares	0	0	0	0
Diluted earnings per share denominator	6,010,200	6,081,100	6,063,200	6,118,400
Basic and diluted earnings per share (in dollars):	$ 1.10	$ 0.06	$ 1.43	$ 0.78
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements				125,000