Discontinued Operations (Earnings Per Share) (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Mar. 31, 2012
Basic and diluted earnings (loss) per share numerators (in millions):
Net income (loss) attributable to White Mountains’ common shareholders	$ 119.9		$ 103.1
Income (Loss) Allocated to Participating Unvested Restricted Common Shares	(1.5)		(1.1)
Net income attributable to White Mountains' common shareholders, net of restricted common share amounts	120.4		93.8
Basic earnings (loss) per share denominators (in thousands):
Total average common shares outstanding during the period	6,273,500		7,431,700
Average unvested restricted common shares (3)	79,000	[1]	78,300	[1]
Basic earnings (loss) per share denominator	6,194,500		7,353,400
Diluted earnings (loss) per share denominator (in thousands):
Total average common shares outstanding during the period	6,273,500		7,431,700
Average unvested restricted common shares (3)	79,000	[1]	78,300	[1]
Average outstanding dilutive options to acquire common shares (4)	0	[2]	0	[2]
Diluted earnings (loss) per share denominator	6,194,500		7,353,400
Segment, Discontinued Operations
Basic and diluted earnings (loss) per share numerators (in millions):
Net income (loss) attributable to White Mountains’ common shareholders	0.5		(9.3)
Income (Loss) Allocated to Participating Unvested Restricted Common Shares	0	[3]	0.1	[3]
Net income attributable to White Mountains' common shareholders, net of restricted common share amounts	$ 0.5	[4]	$ (9.2)	[4]
Basic earnings (loss) per share denominators (in thousands):
Total average common shares outstanding during the period	6,273,500		7,431,700
Average unvested restricted common shares (3)	79,000		78,300	[5]
Basic earnings (loss) per share denominator	6,194,500		7,353,400
Diluted earnings (loss) per share denominator (in thousands):
Total average common shares outstanding during the period	6,273,500		7,431,700
Average unvested restricted common shares (3)	79,000		78,300	[5]
Average outstanding dilutive options to acquire common shares (4)	0	[2]	0	[2]
Diluted earnings (loss) per share denominator	6,194,500		7,353,400
Basic and diluted earnings (loss) per share (in dollars):	$ 0.07		$ (1.25)

[1] Restricted shares outstanding vest either in equal annual installments or upon a stated date (see Note 13).

[2] The diluted earnings per share denominator for the three months ended March 31, 2013 and 2012 do not include common shares issuable upon exercise of the Non-Qualified Options as they are anti-dilutive to the calculation.

[3] Restricted shares issued by White Mountains contain dividend participation features, and therefore, are considered participating securities.

[4] Net earnings (loss) attributable to White Mountains’ common shareholders, net of restricted share amounts, is equal to undistributed earnings (loss) for the three months ended March 31, 2013 and 2012.

[5] Restricted common shares outstanding vest either in equal annual installments or upon a stated date (see Note 13).