Discontinued Operations (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Basic and diluted earnings per share numerators (in millions):
Allocation of income for participating unvested restricted common shares									$ (2.3)	$ (0.7)	$ (2.6)
Net income attributable to White Mountains’s common shareholders	$ 267.9	$ (58.9)	$ 4.3	$ 84.3	$ 69.9	$ 51.3	$ 95.5	$ 95.5	$ 297.6	$ 312.2	$ 321.6
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									5,879,200	6,104,900	6,200,400
Average unvested restricted common shares									(68,000)	(78,900)	(91,400)
Basic earnings per share denominator									5,811,200	6,026,000	6,109,000
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period									5,879,200	6,104,900	6,200,400
Average unvested restricted common shares									(68,000)	(78,900)	(91,400)
Average outstanding dilutive options to acquire common shares									0	0	0
Diluted earnings per share denominator									5,811,200	6,026,000	6,109,000
Incremental common shares attributable to share-based payment arrangements									125,000	125,000	125,000
Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Total income from discontinued operations									$ 98.8	$ 258.6	$ 146.5
Basic and diluted earnings per share numerators (in millions):
Allocation of income for participating unvested restricted common shares									(1.1)	3.3	(2.2)
Net income attributable to White Mountains’s common shareholders									$ 97.7	$ 255.3	$ 144.3
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									5,879,200	6,104,900	6,200,400
Average unvested restricted common shares									(68,000)	(78,900)	(91,400)
Basic earnings per share denominator									5,811,200	6,026,000	6,109,000
Diluted earnings per share denominator (in thousands):
Total average common shares outstanding during the period									5,879,200	6,104,900	6,200,400
Average unvested restricted common shares									(68,000)	(78,900)	(91,400)
Average outstanding dilutive options to acquire common shares									0	0	0
Diluted earnings per share denominator									5,811,200	6,026,000	6,109,000
Basic and diluted earnings per share (in dollars):									$ 16.80	$ 42.36	$ 23.63
Incremental common shares attributable to share-based payment arrangements									125,000	125,000	125,000