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Employee Share-Based Incentive Compensation Plans (OneBeacon - Performance Shares Outstanding) (Details) (USD $) In Millions, except Share data, unless otherwise specified | Mar. 31, 2013 | Dec. 31, 2012 | Mar. 31, 2012 | Dec. 31, 2011
--- | --- | --- | --- | ---
Share-based compensation arrangement by share-based payment award |  |  |  |
Accrued incentive compensation | $ 100.8 | $ 159.0 |  |
Performance Share | One Beacon Long Term Incentive Plan |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award Equity Instruments Other than Options Nonvested, Number before Forfeitures | 502,428 |  |  |
Incentive Compensation Payable before Forfeitures | 1.6 |  |  |
Target Performance Shares Outstanding, Assumed forfeitures | (12,561) |  |  |
Accrued Expense, Assumed forfeitures | 0 |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number | 489,867 | 563,190 | 569,745 | 642,667
Accrued incentive compensation | 1.6 | 1.2 | 3.2 | 9.7
Performance Share | One Beacon Long Term Incentive Plan | Performance Cycle 2013 to 2015 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award Equity Instruments Other than Options Nonvested, Number before Forfeitures | 179,000 |  |  |
Incentive Compensation Payable before Forfeitures | 0.2 |  |  |
Performance Share | One Beacon Long Term Incentive Plan | Performance Cycle 2012 To 2014 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award Equity Instruments Other than Options Nonvested, Number before Forfeitures | 181,290 |  |  |
Incentive Compensation Payable before Forfeitures | 1.1 |  |  |
Performance Share | One Beacon Long Term Incentive Plan | Performance Cycle 2011 to 2013 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award Equity Instruments Other than Options Nonvested, Number before Forfeitures | 142,138 |  |  |
Incentive Compensation Payable before Forfeitures | $ 0.3 |  |  |

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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 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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