Discontinued Operations (Earnings Per Share) (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Basic and diluted earnings (loss) per share numerators (in millions):
Allocation of income for participating unvested restricted common shares									$ (4.1)	$ (4.7)	$ (4.3)
Net income attributable to White Mountains’s common shareholders	70.0	51.5	95.6	95.6	117.9	57.2	26.3	120.4	312.7	321.8	207.4
Basic earnings (loss) per share denominators (in thousands):
Total average common shares outstanding during the period									6,104,900	6,200,400	6,799,800
Average unvested restricted common shares									(78,900)	(91,400)	(91,100)
Basic earnings (loss) per share denominator									6,026,000	6,109,000	6,708,700
Diluted earnings (loss) per share denominator (in thousands):
Total average common shares outstanding during the period									6,104,900	6,200,400	6,799,800
Average unvested restricted common shares									(78,900)	(91,400)	(91,100)
Average outstanding dilutive options to acquire common shares									0	0	0
Diluted earnings (loss) per share denominator									6,026,000	6,109,000	6,708,700
Incremental common shares attributable to share-based payment arrangements									125,000
Discontinued Operations
Discontinued Operations
Net (loss) income from discontinued operations									(3.4)	4.5	(115.0)
Basic and diluted earnings (loss) per share numerators (in millions):
Allocation of income for participating unvested restricted common shares									0	0	1.5
Net income attributable to White Mountains’s common shareholders									$ (3.4)	$ 4.5	$ (113.5)
Basic earnings (loss) per share denominators (in thousands):
Total average common shares outstanding during the period									6,104,900	6,200,400	6,799,800
Average unvested restricted common shares									(78,900)	(91,400)	(91,100)
Basic earnings (loss) per share denominator									6,026,000	6,109,000	6,708,700
Diluted earnings (loss) per share denominator (in thousands):
Total average common shares outstanding during the period									6,104,900	6,200,400	6,799,800
Average unvested restricted common shares									(78,900)	(91,400)	(91,100)
Average outstanding dilutive options to acquire common shares									0	0	0
Diluted earnings (loss) per share denominator									6,026,000	6,109,000	6,708,700
Basic and diluted earnings (loss) per share (in dollars):									$ (0.56)	$ 0.74	$ (16.91)
Incremental common shares attributable to share-based payment arrangements										125,000