Significant Transactions (Details) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended						3 Months Ended		3 Months Ended			2 Months Ended	1 Months Ended				3 Months Ended	9 Months Ended	3 Months Ended	2 Months Ended			3 Months Ended	2 Months Ended
	Sep. 30, 2014 USD ($)	Sep. 30, 2013 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2013 USD ($)	Dec. 31, 2013 USD ($)	Jul. 01, 2014 durchblicker.at [Member]	Sep. 30, 2014 Star & Shield Insurance Exchange USD ($)	Jan. 31, 2013 Star & Shield Insurance Exchange USD ($)	Mar. 31, 2014 QuoteLab USD ($)	Mar. 14, 2014 QuoteLab USD ($)	Mar. 31, 2013 OneBeacon Essentia USD ($)	Mar. 31, 2014 WM Solutions [Member] Citation [Member] USD ($)	Mar. 31, 2013 WM Solutions [Member] Ashmere USD ($)	Feb. 19, 2014 WOBI business	Jan. 31, 2014 Star & Shield LLC [Member] USD ($)	Sep. 30, 2014 Common Class A QuoteLab	Mar. 14, 2014 Common Class A QuoteLab	Feb. 19, 2014 Common Stock WOBI	Sep. 30, 2013 Non-controlling interest Star & Shield Insurance Exchange USD ($)	Sep. 30, 2014 Non-controlling interest Star & Shield Insurance Exchange USD ($)	Sep. 30, 2014 Euro Member Countries, Euro Common Stock durchblicker.at [Member] ILS	Feb. 19, 2014 Israel, New Shekels Preferred Stock WOBI ILS	Feb. 19, 2014 Israel, New Shekels Common Stock WOBI ILS	Feb. 19, 2014 United States of America, Dollars Preferred Stock WOBI USD ($)	Sep. 30, 2014 United States of America, Dollars Common Stock durchblicker.at [Member] ILS	Feb. 19, 2014 United States of America, Dollars Common Stock WOBI USD ($)	Feb. 19, 2014 WOBI USD ($)	Jun. 30, 2014 PassportCard [Member] USD ($)	Jun. 30, 2014 PassportCard [Member] White Mountains	Jun. 30, 2014 PassportCard [Member] DavidShield [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Ownerhsip Percentage of Common Equity						45.00%
Other Commitment																												$ 21.0
Share of joint venture																													50	50
Ownership interest (as a percent)														60.70%		58.40%	60.00%	54.00%
Payments to Acquire Businesses and Interest in Affiliates			32.2	9.2					28.1						1.8						8.5	12.7	14.4	3.6	11.7	4.1
Premiums Written, Percentage of Total														85.00%
Number of lines of business														4
Purchase of Surplus Notes								17.0
Percentage of gross profit										62.50%
Assets	12,188.3		12,188.3		12,144.3		11.8			56.9																	13.4
Intangible Assets, Net (Including Goodwill)										43.7
Intangible Assets, Net																											8.4
Liabilities	7,689.5		7,689.5		7,746.9		22.8			10.0																	0.7
Net Income (Loss) Attributable to Noncontrolling Interest	11.2	1.1	7.1	12.7															1.4	11.1
Gain on sale of subsidiary			0.7	23.0							23.0	0.7	6.9
Gain on sale of subsidiary, after tax											$ 15.0