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Employee Share-Based Incentive Compensation Plans (Details 6) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended | 3 Months Ended | 12 Months Ended | 1 Months Ended |  | 1 Months Ended | 12 Months Ended |
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Dec. 31, 2011 Qualified Plan | Dec. 31, 2010 Qualified Plan | Dec. 31, 2009 Qualified Plan | Mar. 31, 2008 OneBeacon Long-Term Incentive Plan Restricted Stock Units, RSU | Dec. 31, 2011 OneBeacon Long-Term Incentive Plan Restricted Stock Units, RSU | Dec. 31, 2010 OneBeacon Long-Term Incentive Plan Restricted Stock Units, RSU | Dec. 31, 2009 OneBeacon Long-Term Incentive Plan Restricted Stock Units, RSU | Dec. 31, 2011 OneBeacon Long-Term Incentive Plan OneBeacon Restricted Stock | May 31, 2011 OneBeacon Long-Term Incentive Plan OneBeacon Restricted Stock Chairman and CEO Y | May 25, 2011 OneBeacon Long-Term Incentive Plan OneBeacon Restricted Stock Chairman and CEO | May 31, 2011 OneBeacon Long-Term Incentive Plan Performance shares | Dec. 31, 2011 OneBeacon Long-Term Incentive Plan Performance shares | Dec. 31, 2010 OneBeacon Long-Term Incentive Plan Performance shares | Dec. 31, 2009 OneBeacon Long-Term Incentive Plan Performance shares | Dec. 31, 2008 OneBeacon Long-Term Incentive Plan Performance shares
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
New grants (in shares) |  |  |  | 116,270 |  |  |  |  |  |  |  | 194,900 | 270,691 | 379,085 |
Proportion of award vests on each of Nov 9, 2009, 2010 and 2011 |  |  |  | 0.33 |  |  |  |  |  |  |  |  |  |  |
Minimum percentage of growth in book value per share for vesting each tranche of units |  |  |  | 4.00% |  |  |  |  |  |  |  |  |  |  |
Expense recognized |  |  |  |  | $ 0.1 | $ 0.5 | $ 0.6 |  |  |  |  | $ 2.2 | $ 10.2 | $ 11.0 |
Outstanding (in shares) |  |  |  |  |  |  |  |  |  |  |  | 642,667 | 1,464,295 | 2,224,215 | 2,212,313
One Beacon Restricted shares issued (in shares) |  |  |  |  |  |  |  |  |  | 630,000 |  |  |  |  |
Numerator value of the ratio in which restricted shares vest |  |  |  |  |  |  |  |  | 1 |  |  |  |  |  |
Denominator value of the ratio in which restricted shares vest |  |  |  |  |  |  |  |  | 4 |  |  |  |  |  |
Forfeiture of performance shares granted for the 2011 - 2013 performance cycle (in shares) |  |  |  |  |  |  |  |  |  |  | 35,000 | 80,378 | 141,017 | 229,783 |
Period in which performance share awards will be reduced by a similar number of shares (in years) |  |  |  |  |  |  |  |  | 5 |  |  |  |  |  |
Noncontrolling interest expense recognized for the dilution of ownership |  |  |  |  |  |  |  | 7.7 |  |  |  |  |  |  |
Variable contribution amounts earned by eligible participants of the KSOP as a percentage of salary | 2.00% | 0.00% | 4.00% |  |  |  |  |  |  |  |  |  |  |  |
Compensation expense to pay benefits and allocate common shares | $ 6.3 | $ 3.0 | $ 11.0 |  |  |  |  |  |  |  |  |  |  |  |
Total common shares outstanding owned by the plans (as a percent) | 3.00% | 3.00% |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition The compensation expense recognized during the period pertaining to the deferred compensation arrangement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 710 -SubTopic 10 -Section 30 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6409875&loc=d3e20015-108363 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph 64, 65 -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 AICPA -Name Accounting Principles Board Opinion (APB) -Number 12 -Paragraph 6, 7 -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 4: 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 + Details
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- Definition Equity-based compensation cost during the period with respect to the award, which will be recognized in income (as well as the total recognized tax benefit) or capitalized as part of the cost of an asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6406099&loc=d3e25284-112666 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph 64, 65 -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 4: 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 5: 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 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(2)(e) -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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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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 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. Reference 2: 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 + Details
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- Definition The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(2)(a) -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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph b(2)(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 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The maximum number of shares (or other type of equity) originally approved (usually by shareholders and board of directors), net of any subsequent amendments and adjustments, for awards under the equity-based compensation plan. As stock or unit options and equity instruments other than options are awarded to participants, the shares or units remain authorized and become reserved for issuance under outstanding awards (not necessarily vested). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph a -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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Represents the variable contribution amounts earned by eligible participants of the KSOP as a percentage of salary. + References No definition available. + Details
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- Definition Represents the noncontrolling interest expense recognized for the dilution of ownership, which resulted from the issuance of the restricted shares. + References No definition available. + Details
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- Definition Represents the percentage of total common shares outstanding owned by the plans. + References No definition available. + 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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- Definition This element represents the percentage of RSUs that vest on each of November 2009, 2010 and 2011. + References No definition available. + Details
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- Definition Represents the numerator value of the ratio in which shares will vest. + References No definition available. + Details
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- Definition Represents the number of years that the CEO of a subsidiary will forfeit performance shares of that subsidiary. + References No definition available. + Details
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