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Employee Share-Based Incentive Compensation Plans (OneBeacon Performance Shares Outstanding) (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Share-based compensation arrangement by share-based payment award |  |  |  |
Accrued incentive compensation | $ 141.7 | $ 108.1 |  |
OneBeacon | Performance Shares | Long Term Incentive Plan |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 456,669 |  |  |
Incentive compensation payable before forfeitures | $ 1.4 |  |  |
forfeitures | (7,234) |  |  |
Assumed forfeitures in period amount | 0.0 |  |  |
Shares outstanding (in shares) | 449,435 | 517,470 | 493,421 | 563,190
Accrued incentive compensation | $ 1.4 | $ 3.4 | $ 4.0 | $ 1.2
Unamortized grant date fair value | $ 1.3 |  |  |
Targeted performance goal for personnel expressed as percentage of growth in intrinsic business value per share | 13.00% | 14.00% | 13.00% |
Range Of Performance Cycle From 2015 to 2017 [Member] | OneBeacon | Performance Shares | Long Term Incentive Plan |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 146,659 |  |  |
Incentive compensation payable before forfeitures | $ 0.7 |  |  |
Range Of Performance Cycle From 2014-2016 [Member] | OneBeacon | Performance Shares | Long Term Incentive Plan |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 142,710 |  |  |
Incentive compensation payable before forfeitures | $ 0.1 |  |  |
Performance Cycle From 2013 to 2015 | OneBeacon | Performance Shares | Long Term Incentive Plan |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Options nonvested, number before forfeitures (in shares) | 167,300 |  |  |
Incentive compensation payable before forfeitures | $ 0.6 |  |  |
Maximum | OneBeacon | Performance Shares | Long Term Incentive Plan |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 20.00% | 21.00% | 20.00% |
Percentage of performance shares earned | 200.00% | 200.00% | 200.00% |
Minimum | OneBeacon | Performance Shares | Long Term Incentive Plan |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Performance goal percentage for minimum payout | 6.00% | 7.00% | 6.00% |

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- Definition Unrecognized cost of unvested share-based compensation awards. + 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 (i) -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 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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