Significant Transactions Significant Transactions (Acquisitions) (Details) ₪ in Millions, $ in Millions			1 Months Ended		2 Months Ended				3 Months Ended								12 Months Ended
	Nov. 06, 2015 USD ($)	Oct. 10, 2014 USD ($)	Sep. 30, 2015 USD ($)	Jan. 31, 2014 USD ($)	Feb. 23, 2015 ILS (₪)	Feb. 23, 2015 USD ($)	Feb. 19, 2014 ILS (₪)	Feb. 19, 2014 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 ILS (₪)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Sep. 01, 2015 USD ($)	Mar. 14, 2014 USD ($)	Dec. 31, 2012 USD ($)
Unusual or Infrequent Item [Line Items]
Assets									$ 10,284.5				$ 10,455.7					$ 10,284.5	$ 10,455.7
Liabilities									5,916.5				5,917.3					5,916.5	5,917.3
White Mountains’s common shareholders’ equity, including portion attributable to NCI									4,368.0				4,538.4					4,368.0	4,538.4
Other assets									318.7				348.7					318.7	348.7
Goodwill									187.9				168.9					187.9	168.9	$ 0.0
Intangible Assets, Net (Excluding Goodwill)									187.8				182.3					187.8	182.3	5.1
Payments to Acquire Businesses and Interest in Affiliates																		37.1	190.1	0.0
Non-controlling interest in consolidated subsidiaries									6.1	$ (16.0)	$ (0.9)	$ (7.3)	(15.0)	$ (11.2)	$ 4.6	$ (0.6)		(18.1)	(22.2)	$ (12.5)
Cashboard [Member]
Unusual or Infrequent Item [Line Items]
Liabilities						$ 1.2
Ownership interest (as a percent)						56.20%
Goodwill						$ 0.3
Cancerinsurance.com [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates	$ 3.1
Cash	1.1
Liabilities	$ 2.0
Business Combination, Contingent Consideration, Liability									6.8									6.8
Tranzact [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates		$ 177.7
Assets		332.8
Liabilities		$ 108.7
Ownership interest (as a percent)		63.20%
Distributed Earnings		$ 44.2
White Mountains’s common shareholders’ equity, including portion attributable to NCI		281.2
Goodwill		145.1							163.8				145.1					163.8	145.1
Intangible Assets, Net (Excluding Goodwill)		146.3							156.1				142.8					156.1	142.8
Business Combination, Contingent Consideration, Liability		7.4
Tranzutary [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates												$ 12.0
Assets		29.6
Liabilities													$ 4.1						$ 4.1
Ownership interest (as a percent)													36.00%						36.00%
Intangible Assets, Net (Excluding Goodwill)													$ 28.9						$ 28.9
TruBridge [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates			$ 31.0
Assets																					$ 54.5
Liabilities																					$ 4.3
Ownership interest (as a percent)																					100.00%
Goodwill																					$ 18.7
Intangible Assets, Net (Excluding Goodwill)																					$ 28.1
Business Combination, Contingent Consideration, Liability									9.7									9.7
MediaAlpha
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates																$ 28.1
Assets																						$ 70.1
Liabilities																						$ 10.0
Ownership interest (as a percent)																						60.00%
Goodwill									18.3				18.3					18.3	18.3			$ 18.3
Intangible Assets, Net (Excluding Goodwill)									24.4				32.5					$ 24.4	32.5			$ 38.5
Percent of gross profit																	62.50%	62.50%
Business Combination, Contingent Consideration, Liability									$ 7.8				$ 7.9					$ 7.8	$ 7.9
WOBI [Member]
Unusual or Infrequent Item [Line Items]
Liabilities								$ 0.7
Ownership interest (as a percent)									96.10%				63.30%					96.10%	63.30%
Goodwill								$ 5.5	$ 5.8				$ 5.5					$ 5.8	$ 5.5
Intangible Assets, Net (Excluding Goodwill)									4.5				2.5					4.5	2.5
Star & Shield LLC [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates				$ 1.8
Star & Shield Insurance Exchange [Member]
Unusual or Infrequent Item [Line Items]
Purchase of Surplus Notes									4.0				17.0					4.0	17.0
Surplus notes									$ 21.0				$ 17.0					21.0	$ 17.0
HG Global
Unusual or Infrequent Item [Line Items]
Surplus notes																							$ 503.0
Total | WOBI [Member]
Unusual or Infrequent Item [Line Items]
Ownership interest (as a percent)								54.00%
Israel, New Shekels | Preferred stocks | WOBI [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates | ₪							₪ 31.5										₪ 79.6
Israel, New Shekels | Total | Cashboard [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates | ₪					₪ 9.5												26.4
Israel, New Shekels | Total | WOBI [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates | ₪							₪ 14.4										₪ 11.8
United States of America, Dollars | Preferred stocks | WOBI [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates								$ 9.0										20.7
United States of America, Dollars | Total | Cashboard [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates						2.4												6.5
United States of America, Dollars | Total | WOBI [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates								4.1										$ 3.1
WOBI [Member]
Unusual or Infrequent Item [Line Items]
Assets						5.5		13.4
Intangible Assets, Net (Excluding Goodwill)						$ 2.8		$ 2.9
Tangible Assets [Member] | Tranzact [Member]
Unusual or Infrequent Item [Line Items]
Other assets		$ 41.4