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Employee Share-Based Incentive Compensation Plans (OneBeacon Performance Shares Outstanding) (Details) (USD $) In Millions, except Share data, unless otherwise specified |  |  |  |  |  |  | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Dec. 31, 2012 | Dec. 31, 2011 | 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 | Dec. 31, 2009 One Beacon Long Term Incentive Plan | Dec. 31, 2012 Performance Share One Beacon Long Term Incentive Plan | Dec. 31, 2011 Performance Share One Beacon Long Term Incentive Plan | Dec. 31, 2010 Performance Share One Beacon Long Term Incentive Plan | Dec. 31, 2012 Performance Share One Beacon Long Term Incentive Plan Performance Cycle From 2010 to 2012 | Dec. 31, 2012 Performance Share One Beacon Long Term Incentive Plan Performance Cycle From 2011 to 2013 | Dec. 31, 2012 Performance Share One Beacon Long Term Incentive Plan Performance Cycle From 2012 to 2014
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |  |  |  |  |
Options nonvested, number before forfeitures (in shares) |  |  |  |  |  |  | 571,511 |  |  | 238,658 | 151,563 | 181,290
Incentive compensation payable before forfeitures |  |  |  |  |  |  | $ 1.2 |  |  | $ 0 | $ 0.3 | $ 0.9
Assumed rorfeitures in period (in shares) |  |  |  |  |  |  | (8,321) |  |  |  |  |
Assumed forfeitures in period amount |  |  |  |  |  |  | 0 |  |  |  |  |
Shares outstanding (in shares) |  |  | 563,190 | 642,667 | 1,464,295 | 2,224,215 | 563,190 |  |  |  |  |
Accrued incentive compensation | 159.0 | 187.9 | 1.2 | 9.7 | 18.5 | 15.1 | 1.2 |  |  |  |  |
Additional compensation cost that would be recognized if all outstanding performance shares vested |  |  |  |  |  |  | $ 1.9 |  |  |  |  |
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share |  |  |  |  |  |  | 10.00% | 11.00% | 12.00% |  |  |
Performance goal percentage for minimum payout |  |  |  |  |  |  | 3.00% | 4.00% | 5.00% |  |  |
Performance goal percentage for maximum payout |  |  |  |  |  |  | 17.00% | 18.00% | 19.00% |  |  |
Payout for maximum growth target percentage |  |  |  |  |  |  | 200.00% | 200.00% | 200.00% |  |  |

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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 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 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 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. + Details
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- Details
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- Definition Aggregate carrying value as of the balance sheet date of liabilities for incentive compensation arrangements. + References No definition available. + Details
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- Definition Carrying value as of the balance sheet date for performance shares before forfeitures. + References No definition available. + Details
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- Definition This element represents additional compensation cost that would be recognized if all outstanding performance shares are vested. + References No definition available. + Details
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- Definition This element represents the reduction for assumed forfeitures to the number of performance shares outstanding. + References No definition available. + Details
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- Definition This element represents the reduction to accrued expense for assumed forfeitures of performance shares. + References No definition available. + Details
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- Definition This element represents the number of non-vested performance shares before forfeiture assumptions. + References No definition available. + Details
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- Definition Represents the lpercentage of target achieved, high end of range. + References No definition available. + Details
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- Definition Represents the percentage growth in intrinsic business value per share to achieve a maximum payment. + References No definition available. + Details
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- Definition Represents the percentage growth in intrinsic business value per share to achieve a minimum payment. + References No definition available. + Details
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- Definition Represents the targeted percentage of growth in intrinsic business value per share to achieve 100% payment of outstanding performance shares. + References No definition available. + Details
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