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Employee Share-Based Incentive Compensation Plans (Details 6) (OneBeacon Long-Term Incentive Plan, USD $) In Millions, except Share data, unless otherwise specified | 1 Months Ended |  | 3 Months Ended | 1 Months Ended | 3 Months Ended | 6 Months Ended |  |  |  |  | 3 Months Ended | 6 Months Ended
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May 31, 2011 OneBeacon Restricted Stock Chairman and CEO | May 25, 2011 OneBeacon Restricted Stock Chairman and CEO | Jun. 30, 2011 OneBeacon Restricted Stock | May 31, 2011 Performance shares | Jun. 30, 2011 Performance shares | Jun. 30, 2010 Performance shares | Jun. 30, 2011 Performance shares | Jun. 30, 2010 Performance shares | Mar. 31, 2011 Performance shares | Dec. 31, 2010 Performance shares | Mar. 31, 2010 Performance shares | Dec. 31, 2009 Performance shares | Mar. 31, 2008 Restricted Stock Units, RSU | Jun. 30, 2011 Restricted Stock Units, RSU | Jun. 30, 2010 Restricted Stock Units, RSU
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
One Beacon Restricted shares issued (in shares) |  | 630,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Forfeiture of performance shares granted for the 2011 - 2013 performance cycle (in shares) |  |  |  | 35,000 | 46,445 | 48,363 | 51,318 | 50,308 |  |  |  |  |  |  |
Total number of performance share grants to be reduced (in shares) | 210,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Numerator value of the ratio in which restricted shares vest | 1 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Denominator value of the ratio in which restricted shares vest | 4 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Period in which performance share awards will be reduced by a similar number of shares (in years) | 5 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expense recognized |  |  |  |  | $ 1.9 | $ (0.6) | $ 2.3 | $ 2.8 |  |  |  |  |  | $ 0.1 | $ 0.2
Expense not yet recognized |  |  | 8.5 |  |  |  |  |  |  |  |  |  |  |  |
Noncontrolling interest expense recognized for the dilution of ownership |  |  | $ 6.5 |  |  |  |  |  |  |  |  |  |  |  |
New grants (in shares) |  |  |  |  |  | 6,449 | 194,900 | 278,860 |  |  |  |  | 116,270 |  |
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 (as a percent) |  |  |  |  |  |  |  |  |  |  |  |  | 4.00% |  |
Outstanding (in shares) |  |  |  |  | 671,727 | 1,770,423 | 671,727 | 1,770,423 | 718,172 | 1,464,295 | 1,812,337 | 2,224,215 |  | 23,057 |
Vesting period if Chairman and CEO was terminated other than for cause, was demoted or suffered a material reduction in compensation (in months) | within 24 months |  |  |  |  |  |  |  |  |  |  |  |  |  |

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- Definition Description of the period of time over which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, which may be expressed in a variety of ways (for example, in years, month and year). + 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)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + 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 noncontrolling interest expense recognized for the dilution of ownership, which resulted from the issuance of the restricted shares. + 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. + Details
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- Definition This element represents the percentage of RSUs that vest on each of November 2009, 2010 and 2011. + Details
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- Definition Represents the denominator value of the ratio in which shares will vest. + Details
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- Definition Represents the numerator value of the ratio in which shares will vest. + Details
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- Definition Represents the total number of performance share grants that will be reduced. + Details
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- Definition Represents the number of years that the CEO of a subsidiary will forfeit performance shares of that subsidiary. + Details
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