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Employee Share-Based Incentive Compensation Plans (OneBeacon - Performance Shares Outstanding) (Details) - USD ($) $ in Millions | Sep. 30, 2016 | Jun. 30, 2016 | Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Dec. 31, 2014
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Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Accrued incentive compensation | $ 124.2 |  | $ 140.7 |  |  |
OneBeacon [Member] | Performance Share | One Beacon Long Term Incentive Plan |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 452,519 |  |  |  |  |
Incentive Compensation Payable before Forfeitures | $ 1.3 |  |  |  |  |
Target Performance Shares Outstanding, Assumed forfeitures | (11,313) |  |  |  |  |
Accrued Expense, Assumed forfeitures | 0.0 |  |  |  |  |
Total Target Performance Shares Outstanding | 441,206 | 441,206 | 449,435 | 462,147 | 462,147 | 517,470
Accrued incentive compensation | $ 1.3 | $ 1.0 | $ 1.4 | $ 2.1 | $ 2.4 | $ 3.4
OneBeacon [Member] | Performance Share | One Beacon Long Term Incentive Plan | Performance Cycle 2016 to 2018 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 163,150 |  |  |  |  |
Incentive Compensation Payable before Forfeitures | $ 0.6 |  |  |  |  |
OneBeacon [Member] | Performance Share | One Beacon Long Term Incentive Plan | Performance Cycle 2015 to 2017 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 146,659 |  |  |  |  |
Incentive Compensation Payable before Forfeitures | $ 0.7 |  |  |  |  |
OneBeacon [Member] | Performance Share | One Beacon Long Term Incentive Plan | Performance Cycle 2014 to 2016 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding | 142,710 |  |  |  |  |
Incentive Compensation Payable before Forfeitures | $ 0.0 |  |  |  |  |

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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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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + 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 number of non-vested performance shares before forfeiture assumptions. + References No definition available. + Details
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