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Held for Sale and Discontinued Operations (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders	$ (137.5)	$ 40.8	$ 3.5	$ (48.0)	$ 22.5	$ 562.1	$ 13.6	$ 29.0	$ (141.2)	$ 627.2	$ 401.8
Net income (loss) attributable to White Mountains’ common shareholders									(124.0)	49.7	(121.6)
Total income (loss) from discontinued operations									(17.2)	577.5	523.4
Allocation of earnings (losses) to participating restricted common shares									1.4	(0.7)	1.5
Basic and diluted (losses) earnings per share numerators									$ (122.6)	$ 49.0	$ (120.1)
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									3,382,500	4,293,800	5,014,900
Average unvested restricted common shares									(40,100)	(54,300)	(64,800.00)
Basic earnings (losses) per share denominator									3,342,400	4,239,500	4,950,100
Diluted earnings per share denominator (in thousands):
Average outstanding dilutive options to acquire common shares									3,382,500	4,293,800	5,018,100
Average unvested restricted common shares									(40,100)	(54,300)	(64,800.00)
Diluted earnings (loss) per share denominator									3,342,400.00	4,239,500	4,953,300.00
Basic (losses) earnings per share (in usd per share)	$ (43.24)	$ 12.83	$ 1.02	$ (12.82)	$ 6.00	$ 130.81	$ 2.97	$ 6.34	$ (41.76)	$ 146.06	$ 80.11
Diluted (losses) earnings per share (in usd per share)	$ (43.24)	$ 12.83	$ 1.02	$ (12.82)	$ 6.00	$ 130.81	$ 2.97	$ 6.34	$ (41.76)	$ 146.06	$ 80.06
Incremental common shares attributable to share-based payment arrangements											3,217
Discontinued Operations
Basic and diluted earnings per share numerators (in millions):
Net income attributable to White Mountains’s common shareholders									$ (141.2)	$ 627.2	$ 401.8
Net income (loss) attributable to White Mountains’ common shareholders									(124.0)	49.7	(121.6)
Total income (loss) from discontinued operations									(17.2)	577.5	523.4
Allocation of earnings (losses) to participating restricted common shares									0.2	(7.3)	(6.8)
Basic and diluted (losses) earnings per share numerators									$ (17.0)	$ 570.2	$ 516.6
Basic earnings per share denominators (in thousands):
Total average common shares outstanding during the period									3,382,500	4,293,800	5,014,900
Average unvested restricted common shares									(40,100)	(54,300)	(64,800)
Basic earnings (losses) per share denominator									3,342,400	4,239,500	4,950,100
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Average outstanding dilutive options to acquire common shares									3,382,500	4,293,800	5,018,100
Average unvested restricted common shares									(40,100)	(54,300)	(64,800)
Diluted earnings (loss) per share denominator									3,342,400	4,239,500	4,953,300
Basic (losses) earnings per share (in usd per share)									$ (5.09)	$ 134.50	$ 104.37
Diluted (losses) earnings per share (in usd per share)									$ (5.09)	$ 134.50	$ 104.32
Incremental common shares attributable to share-based payment arrangements											40,000

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