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Employee Share-Based Incentive Compensation Plans (OneBeacon - Performance Shares Outstanding) (Details) (USD $) In Millions, except Share data, unless otherwise specified | Sep. 30, 2014 | Jun. 30, 2014 | Dec. 31, 2013 | Sep. 30, 2013 | Jun. 30, 2013 | Dec. 31, 2012
--- | --- | --- | --- | --- | --- | ---
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Accrued incentive compensation | $ 187.5 |  | $ 218.3 |  |  |
OneBeacon | Performance Share | One Beacon Long Term Incentive Plan |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 526,090 |  |  |  |  |
Incentive Compensation Payable before Forfeitures | 5.6 |  |  |  |  |
Target Performance Shares Outstanding, Assumed forfeitures | (13,152) |  |  |  |  |
Accrued Expense, Assumed forfeitures | (0.1) |  |  |  |  |
Total Target Performance Shares Outstanding | 512,938 | 512,938 | 493,421 | 489,867 | 489,867 | 563,190
Accrued incentive compensation | 5.5 | 4.4 | 4.0 | 2.5 | 2.4 | 1.2
Performance Cycle 2012 To 2014 | OneBeacon | Performance Share | One Beacon Long Term Incentive Plan |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 181,290 |  |  |  |  |
Incentive Compensation Payable before Forfeitures | 2.8 |  |  |  |  |
Performance Cycle 2014 to 2016 | OneBeacon | Performance Share | One Beacon Long Term Incentive Plan |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 165,800 |  |  |  |  |
Incentive Compensation Payable before Forfeitures | 0.6 |  |  |  |  |
Performance Cycle 2013 to 2015 | OneBeacon | Performance Share | One Beacon Long Term Incentive Plan |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 179,000 |  |  |  |  |
Incentive Compensation Payable before Forfeitures | $ 2.2 |  |  |  |  |

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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 + Details
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Period Type: | instant

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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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