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Employee Share-Based Incentive Compensation Plans (Details 2) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009 | Dec. 31, 2008
Share-based compensation arrangement by share-based payment award |  |  |  |
Accrued incentive compensation | $ 187.9 | $ 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 | 60.3 |  |  |
Assumed forfeitures (in shares) | (3,524) |  |  |
Assumed forfeitures | (1.6) |  |  |
Target Performance Shares Outstanding | 137,451 |  |  |
Accrued incentive compensation | 58.7 |  |  |
WTM Incentive Plan | Performance cycle 2009-2011 | Performance shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target Performance Shares Outstanding (in shares) | 60,410 |  |  |
Accrued Expense | 42.6 |  |  |
Growth in intrinsic business value per share, target (as a percent) | 10.00% |  |  |
Growth in intrinsic business value per share, maximum (as a percent) | 17.00% |  |  |
Percentage of payout for maximum growth target percentage | 200.00% |  |  |
Growth in intrinsic business value per share, minimum (as a percent) | 3.00% |  |  |
WTM Incentive Plan | Performance cycle 2010-2012 | Performance shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target Performance Shares Outstanding (in shares) | 42,890 |  |  |
Accrued Expense | 10.9 |  |  |
Growth in intrinsic business value per share, target (as a percent) | 11.00% |  |  |
Growth in intrinsic business value per share, maximum (as a percent) | 18.00% |  |  |
Percentage of payout for maximum growth target percentage | 200.00% |  |  |
Growth in intrinsic business value per share, minimum (as a percent) | 4.00% |  |  |
WTM Incentive Plan | Performance cycle 2011-2013 | Performance shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target Performance Shares Outstanding (in shares) | 37,675 |  |  |
Accrued Expense | 6.8 |  |  |
Growth in intrinsic business value per share, target (as a percent) | 10.00% |  |  |
Growth in intrinsic business value per share, maximum (as a percent) | 17.00% |  |  |
Percentage of payout for maximum growth target percentage | 200.00% |  |  |
Growth in intrinsic business value per share, minimum (as a percent) | 3.00% |  |  |
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 | 7.5 |  |  |
Assumed forfeitures (in shares) | (324) |  |  |
Assumed forfeitures | (0.1) |  |  |
Target Performance Shares Outstanding | 12,613 |  |  |
Accrued incentive compensation | 7.4 |  |  |
WTM Phantom Share Plans | Performance cycle 2009-2011 | Performance shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target Performance Shares Outstanding (in shares) | 7,947 |  |  |
Accrued Expense | 6.0 |  |  |
WTM Phantom Share Plans | Performance cycle 2010-2012 | Performance shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target Performance Shares Outstanding (in shares) | 4,990 |  |  |
Accrued Expense | 1.5 |  |  |
WTM Phantom Share Plans | Performance cycle 2011-2013 | Performance shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Assumed forfeitures (in shares) | 0 |  |  |
Assumed forfeitures | 0 |  |  |
OneBeacon Long-Term Incentive Plan | Performance shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target Performance Shares Outstanding (in shares) | 652,562 |  |  |
Accrued Expense | 9.8 |  |  |
Assumed forfeitures (in shares) | (9,895) |  |  |
Assumed forfeitures | (0.1) |  |  |
Target Performance Shares Outstanding | 642,667 | 1,464,295 | 2,224,215 | 2,212,313
Accrued incentive compensation | 9.7 | 18.5 | 15.1 | 4.6
Growth in intrinsic business value per share, target (as a percent) | 11.00% | 12.00% |  |
Growth in intrinsic business value per share, maximum (as a percent) | 18.00% | 19.00% |  |
Percentage of payout for maximum growth target percentage | 200.00% | 200.00% |  |
Growth in intrinsic business value per share, minimum (as a percent) | 4.00% | 5.00% |  |
OneBeacon Long-Term Incentive Plan | Performance cycle 2011-2013 | Performance shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target Performance Shares Outstanding (in shares) | 151,563 |  |  |
Accrued Expense | 0.9 |  |  |
OneBeacon Long-Term Incentive Plan | Performance cycle 2009-2011 | Performance shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target Performance Shares Outstanding (in shares) | 256,751 |  |  |
Accrued Expense | 7.7 |  |  |
OneBeacon Long-Term Incentive Plan | Performance cycle 2010-2012 | Performance shares |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target Performance Shares Outstanding (in shares) | 244,248 |  |  |
Accrued Expense | $ 1.2 |  |  |

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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 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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