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Employee Share-Based Incentive Compensation Plans (Details 6) (USD $) | 3 Months Ended |  | 1 Months Ended |  | 3 Months Ended
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Mar. 31, 2012 OneBeacon Restricted Stock | Mar. 31, 2008 OneBeacon Long-Term Incentive Plan Restricted Stock Units, RSU | Mar. 31, 2012 OneBeacon Long-Term Incentive Plan OneBeacon Restricted Stock | Mar. 01, 2012 OneBeacon Long-Term Incentive Plan OneBeacon Restricted Stock installment | 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 installment | Mar. 31, 2012 OneBeacon Long-Term Incentive Plan Performance shares | Mar. 31, 2011 OneBeacon Long-Term Incentive Plan Performance shares
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |
Total number of performance share grants to be reduced |  |  |  |  | 35,000 |  |  |
Number of installments in which the shares vest |  |  |  | 2 |  | 4 |  |
Activity in plan |  |  |  |  |  |  |  |
Beginning of period (in shares) | 630,000 |  |  |  |  |  | 642,667 | 1,464,295
Issued (in shares) | 300,000 |  | 300,000 |  | 630,000 |  |  |
Ending of period (in shares) | 930,000 |  |  |  |  |  | 569,745 | 718,172
Unamortized grant date fair value |  |  |  |  |  |  |  |
Unamortized grant date fair value, beginning balance | $ 7,700,000 |  |  |  |  |  |  |
Issued | 4,600,000 |  |  |  |  |  |  |
Expense recognized | (400,000) |  |  |  |  |  | 1,000,000 | 400,000
Unamortized grant date fair value, ending balance | 11,900,000 |  |  |  |  |  |  |
New grants (in shares) |  | 116,270 |  |  |  |  | 181,290 | 194,900
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 | $ (400,000) |  |  |  |  |  | $ 1,000,000 | $ 400,000
Period in which performance share awards will be reduced by a similar number of shares (in years) |  |  |  |  | 5 |  |  |

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- Definition As of the balance sheet date, the aggregate unrecognized cost of equity-based awards made to employees under equity-based compensation awards that have yet to vest. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph h -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 (i) -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 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 number of shares newly issued during the reporting period under the plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph 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 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -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 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The total fair value (on the date of issue) for restricted share awards issued during the period. + References No definition available. + Details
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- Definition Represents the number of installments in which the shares vest. + 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 total number of performance share grants that will be reduced. + 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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