Significant Transactions Significant Transactions (Acquisitions) (Details) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended			3 Months Ended	0 Months Ended				3 Months Ended				2 Months Ended			1 Months Ended		12 Months Ended				2 Months Ended				3 Months Ended			0 Months Ended	12 Months Ended
	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 30, 2013 USD ($)	Jun. 30, 2013 USD ($)	Mar. 31, 2013 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2014 Olympus [Member] USD ($)	Oct. 10, 2014 Tranzact [Member] USD ($)	Dec. 31, 2014 Tranzact [Member] USD ($)	Oct. 10, 2014 Tranzact [Member] USD ($)	Dec. 31, 2013 Tranzact [Member] USD ($)	Mar. 31, 2014 QuoteLab [Member] USD ($)	Dec. 31, 2014 QuoteLab [Member] USD ($)	Mar. 14, 2014 QuoteLab [Member] USD ($)	Dec. 31, 2013 QuoteLab [Member] USD ($)	Feb. 19, 2014 WOBI [Member] USD ($) business	Dec. 31, 2014 WOBI [Member] USD ($)	Dec. 31, 2013 WOBI [Member] USD ($)	Jan. 31, 2014 Star & Shield LLC [Member] USD ($)	Dec. 31, 2014 Star & Shield Insurance Exchange [Member] USD ($)	Dec. 31, 2013 HG Global-BAM USD ($)	Dec. 31, 2014 HG Global USD ($)	Dec. 31, 2013 HG Global USD ($)	Feb. 19, 2014 Total WOBI [Member]	Feb. 19, 2014 Israel, New Shekels Preferred stocks WOBI [Member] ILS	Feb. 19, 2014 Israel, New Shekels Total WOBI [Member] ILS	Feb. 19, 2014 United States of America, Dollars Preferred stocks WOBI [Member] USD ($)	Feb. 19, 2014 United States of America, Dollars Total WOBI [Member] USD ($)	Mar. 31, 2013 American Fuji [Member] WM Solutions USD ($)	Mar. 31, 2014 Empire [Member] WM Solutions USD ($)	Feb. 19, 2014 WOBI [Member] USD ($)	Dec. 03, 2012 Runoff Entities [Member] WM Solutions USD ($)	Dec. 31, 2014 Non-controlling Interests Star & Shield Insurance Exchange [Member] USD ($)	Dec. 31, 2015 Scenario, Forecast [Member] QuoteLab [Member]	Oct. 10, 2014 Tangible Assets [Member] Tranzact [Member] USD ($)
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates												$ 11.5	$ 177.7				$ 28.1
Assets	10,456.9				12,144.3				10,456.9	12,144.3		24.6			332.8				70.1						13.5											13.4
Liabilities	5,917.3				7,746.9				5,917.3	7,746.9		13.1			108.7				10.0		0.7				25.9
Ownership interest (as a percent)															63.20%				60.00%			63.30%							54.00%
Premiums Written, Percentage of Total																					85.00%
Distributed Earnings													44.2
White Mountains’s common shareholders’ equity, including portion attributable to NCI	4,539.6				4,397.4				4,539.6	4,397.4					281.2
Other assets	356.1				437.4				356.1	437.4																														41.4
Business Combination, Bargain Purchase, Gain Recognized, Amount																																		6.9	7.3		14.5
Goodwill	168.9				0				168.9	0	0			145.1	145.1	0		18.3	18.3	0	5.5	5.5	0
Intangible Assets, Net (Excluding Goodwill)	197.5				20.7				197.5	20.7	16.9			142.8	146.3	0		32.5	38.5	0		2.5	0													2.9
Percent of gross profit																																							62.50%
Business Combination, Contingent Consideration, Liability															7.4				7.9
Payments to Acquire Businesses and Interest in Affiliates									199.2	(15.9)	41.3													1.8						31.5	14.4	9.0	4.1
Number of lines of business																					4
Purchase of Surplus Notes																									17.0
Non-controlling interest in consolidated subsidiaries	(15.0)	(11.2)	4.6	(0.5)	0.3	(1.1)	(11.1)	(0.5)	(22.1)	(12.4)	(14.0)																											(12.4)
Capital contributions																										594.5
Surplus notes																									$ 17.0		$ 503.0	$ 503.0