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Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Stock Units) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended | 1 Months Ended | 12 Months Ended
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Dec. 31, 2012 One Beacon Restricted Stock | Dec. 31, 2011 One Beacon Restricted Stock | Dec. 31, 2012 One Beacon Long Term Incentive Plan | Dec. 31, 2011 One Beacon Long Term Incentive Plan | Dec. 31, 2010 One Beacon Long Term Incentive Plan | Mar. 31, 2008 One Beacon Long Term Incentive Plan One Beacon Restricted Stock | Dec. 31, 2011 One Beacon Long Term Incentive Plan Restricted Stock Units (RSUs) | Dec. 31, 2010 One Beacon Long Term Incentive Plan Restricted Stock Units (RSUs)
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |
New grants (in shares) |  |  | 181,290 | 194,900 | 270,691 | 116,270 |  |
Percentage of growth in book value per share |  |  |  |  |  | 4.00% |  |
Expense recognized | $ 2.6 | $ 0.9 | $ 0.8 | $ (2.2) | $ (10.2) |  | $ 0.1 | $ 0.5

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph g(1) -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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. Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph 64 -Subparagraph b -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. Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=6793087&loc=d3e301413-122809 + Details
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Period Type: | duration

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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(2)(c) -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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- Definition This element represents the minimum percentage of growth in book value per share from January 1, 2008 through the end of the calendar year immediately following the applicable vesting date required to trigger the vesting of restricted stock units. + References No definition available. + Details
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