1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date. 2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Projected Benefit Obligation

-URI http://asc.fasb.org/extlink&oid=6522206

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Accumulated Postretirement Benefit Obligation

-URI http://asc.fasb.org/extlink&oid=6503904

Retirement and Postretirement Plans (Cash Flow) (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Employee savings plan
Defined Benefit Plan, Benefit Obligation	$ 119,700,000	$ 102,900,000	$ 119,500,000
Funded status at end of year	26,300,000	39,900,000
OneBeacon Ltd.
Expected Benefit Payments
2015	4,900,000
2016	5,300,000
2017	5,600,000
2018	5,900,000
2019	6,100,000
2020-2024	34,400,000
Employee savings plan
Anticipated contribution to plan	2,200,000
OneBeacon
Employee savings plan
Post-employment benefit liability	4,800,000	5,800,000
Sirius Group
Employee savings plan
Defined Contribution Plan, Cost Recognized	500,000	700,000	400,000
Government Pension Plans [Member] \| Sirius Group
Employee savings plan
Defined Contribution Plan, Cost Recognized	10,300,000	9,700,000	7,500,000
Defined Benefit Plan, Benefit Obligation	14,100,000	13,100,000
Funded status at end of year	(1,800,000)	(1,000,000)
Qualified Plan [Member] \| Sirius Group
Employee savings plan
Defined Contribution Plan, Cost Recognized	700,000	600,000	600,000
Combination One [Member] \| Sirius Group
Employee savings plan
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	2.00%
Combination Two [Member] \| Sirius Group
Employee savings plan
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	4.00%
Maximum \| Sirius Group
Employee savings plan
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	100.00%
Employer matching contribution variable contributions	7.00%
Minimum \| Sirius Group
Employee savings plan
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	50.00%
One Beacon Employee Stock Ownership Plan
Employee savings plan
Defined Contribution Plan, Cost Recognized	4,400,000	6,300,000	4,800,000
Annual base contribution provided to participants expressed as percentage of salary	3.00%
Social security wage base amount maximum	117,000
Variable contribution provided to participants (as percentage)	6.00%
One Beacon Employee Stock Ownership Plan \| OneBeacon
Employee savings plan
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	50.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	6.00%
Defined Contribution Plan, Cost Recognized	2,700,000	2,300,000	2,800,000
IRS \| OneBeacon
Employee savings plan
Defined Benefit Plan Compensation LImits	260,000
IRS \| Sirius Group
Employee savings plan
Defined Benefit Plan Compensation LImits	$ 260,000