Retirement and Postretirement Plans (Cash Flow) (Details) (USD $)				12 Months Ended				12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2014 OneBeacon Ltd.	Dec. 31, 2013 OneBeacon Ltd.	Dec. 31, 2013 OneBeacon	Dec. 31, 2012 OneBeacon	Dec. 31, 2013 Sirius Group	Dec. 31, 2012 Sirius Group	Dec. 31, 2011 Sirius Group	Dec. 31, 2013 Government Pension Plans [Member] Sirius Group	Dec. 31, 2012 Government Pension Plans [Member] Sirius Group	Dec. 31, 2011 Government Pension Plans [Member] Sirius Group	Dec. 31, 2013 Defined Contribution Plan [Member] Sirius Group	Dec. 31, 2012 Defined Contribution Plan [Member] Sirius Group	Dec. 31, 2011 Defined Contribution Plan [Member] Sirius Group	Dec. 31, 2013 Combination One [Member] Defined Contribution Plan [Member] Sirius Group	Dec. 31, 2013 Combination Two [Member] Defined Contribution Plan [Member] Sirius Group	Dec. 31, 2013 Maximum Sirius Group	Dec. 31, 2013 Maximum Defined Contribution Plan [Member] Sirius Group	Dec. 31, 2013 Minimum Defined Contribution Plan [Member] Sirius Group	Dec. 31, 2013 One Beacon Employee Stock Ownership Plan	Dec. 31, 2012 One Beacon Employee Stock Ownership Plan	Dec. 31, 2011 One Beacon Employee Stock Ownership Plan	Dec. 31, 2013 One Beacon Employee Stock Ownership Plan OneBeacon	Dec. 31, 2012 One Beacon Employee Stock Ownership Plan OneBeacon	Dec. 31, 2011 One Beacon Employee Stock Ownership Plan OneBeacon	Dec. 31, 2013 IRS OneBeacon	Dec. 31, 2013 IRS Sirius Group
Expected Benefit Payments
2014					$ 5,100,000
2015					5,400,000
2016					5,600,000
2017					5,800,000
2018					6,100,000
2019-2023					33,900,000
Employee savings plan
Defined Contribution Plan, Employer Matching Contribution, Percent of Match																				100.00%	50.00%				50.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay																	2.00%	4.00%							6.00%
Defined Contribution Plan, Cost Recognized								700,000	400,000	300,000	9,700,000	7,500,000	9,600,000	600,000	600,000	700,000						6,300,000	4,800,000	6,300,000	2,300,000	2,800,000	3,000,000
Anticipated contribution to plan				2,200,000
Annual base contribution provided to participants expressed as percentage of salary																						3.00%
Social security wage base amount maximum																						113,700
Variable contribution provided to participants (as percentage)																						6.00%
Employer matching contribution variable contributions																			7.00%
Deferred Benefit Plan Compensation Limits																												255,000	255,000
Defined Benefit Plan, Benefit Obligation	102,900,000	119,500,000	113,000,000								13,100,000	12,400,000
Funded status at end of year	39,900,000	5,200,000									1,000,000	1,700,000
Post-employment benefit liability						$ 5,800,000	$ 7,100,000