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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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- Definition The employer's best estimate, as soon as it can be reasonably determined, of contributions expected to be paid to the plan during the next fiscal year beginning after the date of the latest statement of financial position. Estimated contributions may be presented in the aggregate combining (1) contributions required by funding regulations or laws, (2) discretionary contributions, and (3) noncash contributions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph g -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy. + Details
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