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Employee Share-Based Incentive Compensation Plans (Details 2) (USD $) In Millions, except Share data | 9 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2011 | Jun. 30, 2011 | Dec. 31, 2010 | Sep. 30, 2010 | Jun. 30, 2010 | Dec. 31, 2009
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Accrued incentive compensation | $ 169.9 |  | $ 157.4 |  |  |
WTM Incentive Plan | Performance shares |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding (in shares) | 140,975 |  |  |  |  |
Accrued Expense | 50.5 |  |  |  |  |
Assumed forfeitures (in shares) | (3,524) |  |  |  |  |
Assumed forfeitures | (1.3) |  |  |  |  |
Target WTM Performance Shares Outstanding | 137,451 |  |  |  |  |
Accrued incentive compensation | 49.2 |  |  |  |  |
WTM Incentive Plan | Performance shares | Performance cycle 2009-2011 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding (in shares) | 60,410 |  |  |  |  |
Accrued Expense | 38.1 |  |  |  |  |
WTM Incentive Plan | Performance shares | Performance cycle 2010-2012 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding (in shares) | 42,890 |  |  |  |  |
Accrued Expense | 7.9 |  |  |  |  |
WTM Incentive Plan | Performance shares | Performance cycle 2011-2013 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding (in shares) | 37,675 |  |  |  |  |
Accrued Expense | 4.5 |  |  |  |  |
WTM Phantom Share Plans | Performance shares |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding (in shares) | 12,937 |  |  |  |  |
Accrued Expense | 6.4 |  |  |  |  |
Assumed forfeitures (in shares) | (324) |  |  |  |  |
Assumed forfeitures | (0.1) |  |  |  |  |
Target WTM Performance Shares Outstanding | 12,613 |  |  |  |  |
Accrued incentive compensation | 6.3 |  |  |  |  |
WTM Phantom Share Plans | Performance shares | Performance cycle 2009-2011 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding (in shares) | 7,947 |  |  |  |  |
Accrued Expense | 5.2 |  |  |  |  |
WTM Phantom Share Plans | Performance shares | Performance cycle 2010-2012 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding (in shares) | 4,990 |  |  |  |  |
Accrued Expense | 1.2 |  |  |  |  |
OneBeacon Long-Term Incentive Plan | Performance shares |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding (in shares) | 662,222 |  |  |  |  |
Accrued Expense | 9.7 |  |  |  |  |
Assumed forfeitures (in shares) | (10,078) |  |  |  |  |
Assumed forfeitures | (0.1) |  |  |  |  |
Target WTM Performance Shares Outstanding | 652,144 | 671,727 | 1,464,295 | 1,564,829 | 1,770,423 | 2,224,215
Accrued incentive compensation | 9.6 | 10.0 | 18.5 | 14.4 | 15.2 | 15.1
OneBeacon Long-Term Incentive Plan | Performance shares | Performance cycle 2008-2010 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding (in shares) | 259,102 |  |  |  |  |
Accrued Expense | 7.2 |  |  |  |  |
OneBeacon Long-Term Incentive Plan | Performance shares | Performance cycle 2009-2011 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding (in shares) | 251,557 |  |  |  |  |
Accrued Expense | 1.9 |  |  |  |  |
OneBeacon Long-Term Incentive Plan | Performance shares | Performance cycle 2010-2012 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target Performance Shares Outstanding (in shares) | 151,563 |  |  |  |  |
Accrued Expense | $ 0.6 |  |  |  |  |

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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 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. 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 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. + Details
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- Definition Aggregate carrying value as of the balance sheet date of liabilities for incentive compensation arrangements. + Details
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Period Type: | instant

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- Definition Carrying value as of the balance sheet date for performance shares before forfeitures. + Details
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- Definition This element represents the reduction for assumed forfeitures to the number of performance shares outstanding. + Details
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- Definition This element represents the reduction to accrued expense for assumed forfeitures of performance shares. + Details
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- Definition This element represents the number of non-vested performance shares before forfeiture assumptions. + Details
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