Reserves for Unpaid Losses and Loss Adjustment Expenses (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended				12 Months Ended			3 Months Ended			6 Months Ended	12 Months Ended				12 Months Ended																				12 Months Ended				12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2012 Sirius Group	Dec. 31, 2011 Sirius Group	Dec. 31, 2010 Sirius Group	Dec. 31, 2012 Sirius Group	Sep. 30, 2012 Sirius Group	Mar. 31, 2012 Sirius Group	Jun. 30, 2012 Sirius Group	Dec. 31, 2012 Sirius Group Exposure	Dec. 31, 2011 Sirius Group	Dec. 31, 2010 Sirius Group	Dec. 31, 2009 Sirius Group	Dec. 31, 2012 OneBeacon	Dec. 31, 2011 OneBeacon	Dec. 31, 2010 OneBeacon	Dec. 31, 2012 Casualty Runoff Sirius Group	Dec. 31, 2012 Property Sirius Group	Dec. 31, 2012 Marine Energy Sirius Group	Dec. 31, 2012 Trade Credit Sirius Group	Dec. 31, 2012 Aviation Space Sirius Group	Dec. 31, 2012 Accident and Health [Member] Sirius Group	Dec. 31, 2011 Professional Liability [Member] OneBeacon	Dec. 31, 2009 Professional Liability [Member] OneBeacon	Dec. 31, 2011 Other Lines of Business OneBeacon	Dec. 31, 2010 Other Lines of Business OneBeacon	Dec. 31, 2011 Commercial Lines OneBeacon	Dec. 31, 2010 Commercial Lines OneBeacon	Dec. 31, 2009 Commercial Lines OneBeacon	Dec. 31, 2012 Asbestos Issue Sirius Group	Dec. 31, 2011 Asbestos Issue Sirius Group	Dec. 31, 2010 Asbestos Issue Sirius Group	Dec. 31, 2009 Asbestos Issue Sirius Group	Dec. 31, 2012 Environmental Issue Sirius Group	Dec. 31, 2011 Environmental Issue Sirius Group	Dec. 31, 2010 Environmental Issue Sirius Group	Dec. 31, 2009 Environmental Issue Sirius Group	Dec. 31, 2012 WM Solutions Sirius Group	Dec. 31, 2012 WM Solutions Sirius Group
Asbestos and Environmental Loss and Loss Adjustment Expense Reserve Activity
Loss and LAE incurred relating prior year losses	$ 41.9	$ 76.7	$ 93.1		$ 34.5	$ 46.9	$ 57.1									$ 7.4	$ 29.8	$ 36.0	$ 31.5	$ 28.4	$ 11.6	$ 6.8	$ 5.3	$ 4.3	$ 11.5	$ 19.3	$ 4.3	$ 9.2		$ 7.5	$ 7.5
Liability for asbestos and environmental claims, net, period increase (decrease)					46.4
Liability for environmental claims, net, incurred loss													2.0
Increase (decrease) in reserves	1,193.9	1,174.3	1,216.6																										14.0
Net favorable loss reserve development due to property lines						41.2
Net favorable loss reserve development due to 2010 Chilean earthquake						13.1
Unfavorable loss reserve development due to asbestos and environmental reserves additions						12.3
Loss and loss adjustment expenses	1,193.9	1,174.3	1,216.6		543.9	626.0	531.0					543.9	626.0	531.0		650.0	548.3	685.6
Deferred gains recognized							16.3
Accretion of fair value adjustment to net loss and LAE reserves	(10.6)	(8.3)	(8.5)
Un-accreted fair value, adjustment to loss and loss adjustment expense reserves, before tax	5.7
Reinsurance recoverable on unpaid losses	429.1	2,507.3	2,344.0	2,771.5				321.8				321.8				107.3
Liability for asbestos and environmental claims								189.4				189.4	162.7	169.6	164.5																	174.2	146.2	151.5	146.6	15.2	16.5	18.1	17.9
Asbestos and Environmental Loss Three Year Survival Ratio Period					9 years	11 years 1 month 6 days						9 years	11 years 1 month 6 days
Liability for asbestos and environmental claims, gross								228.6				228.6	207.2	214.3	209.2	1.0																208.2	185.1	191.9	187.0	20.4	22.1	22.4	22.2
Liabilty for asbestos claims, net, period increase (decrease)								2.0	33.0		12.0	46.4	10.3																											11.0	11.0
Liabilty for environmental claims, net, period increase (decrease)												0.5																												0.7
Liability for unpaid claims and claims adjustment expense										$ 8.5		$ 30.9	$ 19.2	$ 13.0																		$ 29.4	$ 15.6	$ 12.1		$ 1.5	$ 3.6	$ 0.9
Number of asbestos exposures					5							5