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Retirement and Postretirement Plans (Cash Flow) (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Expected Benefit Payments
2013	$ 5,200,000
2014	5,500,000
2015	5,800,000
2016	6,000,000
2017	6,200,000
2018-2022	35,000,000
One Beacon Segment
Employee savings plan
Percentage of employer's matching contribution for first six percent of salary	50.00%
Maximum percentage of salary eligible for employer's fifty percent matching contribution	6.00%
Total expense for the plan during the period	2,800,000	3,000,000	3,800,000
Employee stock ownership Plan (ESOP), salary amount maximum	250,000
Post-employment benefit liability	7,100,000	6,700,000
Sirius Group
Employee savings plan
Deferred compensation expense	1,000,000	1,100,000	1,000,000
Non-qualified Plan | OneBeacon Ltd.
Defined Benefit Plan Disclosure [Line Items]
Anticipated contribution to plan in 2013	2,400,000
Employee Stock Ownership Plan (ESOP), Plan
Employee savings plan
Deferred compensation expense	4,800,000	6,300,000	3,000,000
Government Pension Plans [Member] | Sirius Group
Employee savings plan
Deferred compensation expense	7,500,000	9,600,000	8,100,000
Match One | Sirius Group
Employee savings plan
Percentage of employer's matching contribution for first six percent of salary	100.00%
Maximum percentage of salary eligible for employer's fifty percent matching contribution	2.00%
Match Two | Sirius Group
Employee savings plan
Percentage of employer's matching contribution for first six percent of salary	50.00%
Maximum percentage of salary eligible for employer's fifty percent matching contribution	4.00%
Maximum | Sirius Group
Employee savings plan
Employer matching contribution variable contributions	7.00%
One Beacon Employee Stock Ownership Plan
Employee savings plan
Annual base contribution provided to participants expressed as percentage of salary	3.00%
Social security wage base amount maximum	$ 110,100
Variable contribution provided to participants not eligible to certain benefits expressed as percentage of salary	6.00%

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The compensation expense recognized during the period pertaining to the deferred compensation arrangement.

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