Significant Transactions Significant Transactions (Acquisitions) (Details) ₪ in Millions, $ in Millions		1 Months Ended	2 Months Ended				3 Months Ended									12 Months Ended
	Aug. 04, 2016 USD ($)	Jan. 31, 2014 USD ($)	Feb. 23, 2015 ILS (₪)	Feb. 23, 2015 USD ($)	Feb. 19, 2014 ILS (₪)	Feb. 19, 2014 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2016 ILS (₪)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 ILS (₪)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Mar. 14, 2014 USD ($)
Unusual or Infrequent Item [Line Items]
Assets							$ 6,544.7				$ 10,282.6						$ 6,544.7		$ 10,282.6
Liabilities							2,807.5				5,914.6						2,807.5		5,914.6
White Mountains’s common shareholders’ equity, including portion attributable to NCI							3,737.2				4,368.0						3,737.2		4,368.0
Other assets							274.6				273.2						274.6		273.2
Goodwill							31.7				24.1						31.7		24.1	$ 23.8
Intangible Assets, Net (Excluding Goodwill)							24.2				31.3						24.2		31.3	38.7
Payments to Acquire Businesses and Interest in Affiliates																	13.4		2.6	205.0
Payments to Acquire Additional Interest in Subsidiaries																	0.0		9.1	0.0
Other revenue																	149.6		147.3	88.1
Non-controlling interest in consolidated subsidiaries							(17.3)	$ (3.1)	$ 21.4	$ 6.3	6.1	$ (16.0)	$ (0.9)	$ (7.3)			7.3		(18.1)	(22.2)
Buzzmove [Member]
Unusual or Infrequent Item [Line Items]
Assets	$ 11.5
Liabilities	$ 0.1
Ownership interest (as a percent)	70.90%
Goodwill	$ 7.6						7.6				0.0						7.6		0.0
Intangible Assets, Net (Excluding Goodwill)	1.1						1.0				0.0						1.0		0.0
Cashboard [Member]
Unusual or Infrequent Item [Line Items]
Liabilities				$ 1.2
Ownership interest (as a percent)				56.20%
Goodwill				$ 0.3
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)							18.3				$ 24.4						18.3		$ 24.4		$ 38.5
Percent of gross profit																		62.50%	62.50%
Additional cash that will be paid as part of the purchase price							$ 7.8										$ 7.8
WOBI [Member]
Unusual or Infrequent Item [Line Items]
Liabilities						$ 0.7
Ownership interest (as a percent)							95.00%				96.10%						95.00%		96.10%
Goodwill						$ 5.5	$ 5.8				$ 5.8						$ 5.8		$ 5.8
Intangible Assets, Net (Excluding Goodwill)							$ 3.7				4.5						3.7		4.5
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								4.0	$ 17.0
Other revenue																	21.0
LowFares.com [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates										$ 3.9
Common Stock | WOBI [Member]
Unusual or Infrequent Item [Line Items]
Ownership interest (as a percent)						54.00%
GBP | Buzzmove [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates	6.1
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 | Common Stock | Cashboard [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Businesses and Interest in Affiliates | ₪			₪ 9.5															26.4
Israel, New Shekels | Common Stock | 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 | Buzzmove [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Interest in Subsidiaries and Affiliates	$ 8.1
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 | Common Stock | 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 | Common Stock | 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
Common Stock | Israel, New Shekels | WOBI [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Additional Interest in Subsidiaries | ₪																₪ 35.9
Common Stock | United States of America, Dollars | WOBI [Member]
Unusual or Infrequent Item [Line Items]
Payments to Acquire Additional Interest in Subsidiaries																	$ 9.6