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Employee Share-Based Incentive Compensation Plans (Details) (USD $) In Millions, except Share data, unless otherwise specified |  |  | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended | 3 Months Ended | 9 Months Ended
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Sep. 30, 2011 | Dec. 31, 2010 | Sep. 30, 2011 WTM Incentive Plan and WTM Phantom Share Plans Performance shares | Sep. 30, 2010 WTM Incentive Plan and WTM Phantom Share Plans Performance shares | Sep. 30, 2011 WTM Incentive Plan and WTM Phantom Share Plans Performance shares | Sep. 30, 2010 WTM Incentive Plan and WTM Phantom Share Plans Performance shares | Sep. 30, 2011 OneBeacon Long-Term Incentive Plan Performance shares | Sep. 30, 2010 OneBeacon Long-Term Incentive Plan Performance shares | Sep. 30, 2011 OneBeacon Long-Term Incentive Plan Performance shares | Sep. 30, 2010 OneBeacon Long-Term Incentive Plan Performance shares | Sep. 30, 2011 OneBeacon Long-Term Incentive Plan Performance shares Performance cycle 2007-2009 | Sep. 30, 2011 OneBeacon Long-Term Incentive Plan Performance shares Performance cycle 2008-2010 | Sep. 30, 2011 OneBeacon Long-Term Incentive Plan Non-Qualified Options | Sep. 30, 2010 OneBeacon Long-Term Incentive Plan Non-Qualified Options | Sep. 30, 2011 OneBeacon Long-Term Incentive Plan Non-Qualified Options | Sep. 30, 2010 OneBeacon Long-Term Incentive Plan Non-Qualified Options | Sep. 30, 2010 Non-Qualified Options | Sep. 30, 2011 Non-Qualified Options | Sep. 30, 2010 Non-Qualified Options
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Vesting period (in years) |  |  |  |  | 3Y |  |  |  |  |  |  |  |  |  | Third, fourth and fifth anniversaries |  |  |  |
Target Performance Share activity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Beginning of period (in shares) |  |  | 150,064 | 163,220 | 163,184 | 172,454 | 671,727 | 1,770,423 | 1,464,295 | 2,224,215 |  |  |  |  |  |  |  |  |
Shares expired (in shares) |  |  |  |  | (51,131) | (49,127) |  | (199,774) | (936,150) | (882,118) |  |  |  |  |  |  |  |  |
New grants (in shares) |  |  |  |  | 37,675 | 47,880 |  | 14,618 | 194,900 | 293,478 |  |  |  |  |  |  |  |  |
Cancellations and change in assumed forfeitures (in shares) |  |  |  | (36) | 336 | (8,023) | (19,583) | (20,438) | (70,901) | (70,746) |  |  |  |  |  |  |  |  |
Ending of period (in shares) |  |  | 150,064 | 163,184 | 150,064 | 163,184 | 652,144 | 1,564,829 | 652,144 | 1,564,829 |  |  |  |  |  |  |  |  |
Expense Amortized |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accrued expense at the beginning of period | $ 169.9 | $ 157.4 | $ 56.5 | $ 20.2 | $ 29.4 | $ 12.2 | $ 10.0 | $ 15.2 | $ 18.5 | $ 15.1 |  |  | $ 4.8 | $ 4.1 | $ 4.5 | $ 3.6 |  |  |
Payments and deferrals |  |  |  |  |  |  |  | (2.2) | (10.5) | (4.5) |  |  |  |  |  |  |  |  |
Assumed forfeitures and cancellations |  |  |  |  | (0.6) |  | 0.1 | (0.3) | (0.2) | (0.7) |  |  |  |  |  |  |  |  |
Expense recognized |  |  | (1.0) | 3.0 | 26.7 | 11.0 | (0.5) | 1.7 | 1.8 | 4.5 |  |  | 0.1 | 0.2 | 0.4 | 0.7 | 10.1 | 0.1 | 11.5
Accrued expense at the end of the period | 169.9 | 157.4 | 55.5 | 23.2 | 55.5 | 23.2 | 9.6 | 14.4 | 9.6 | 14.4 |  |  | 4.9 | 4.3 | 4.9 | 4.3 |  |  |
Percentage of target achieved by former employees as a result of the Commercial Lines and Personal Lines Transactions (as a percent) |  |  |  |  |  |  |  |  | 100.00% |  | 14.20% | 68.50% |  |  |  |  |  |  |
Target vesting percentage of performance shares (as a percent) |  |  | 100.00% |  | 100.00% |  | 100.00% |  | 100.00% |  |  |  |  |  |  |  |  |  |
Additional compensation cost that would be recognized if all outstanding performance shares vested |  |  | $ 23.5 |  | $ 23.5 |  | $ 3.7 |  | $ 3.7 |  |  |  |  |  |  |  |  |  |

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- Definition Description of the period of time over which an employee's right to exercise an award is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, which may be expressed in a variety of ways (for example, in years, month and year). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph 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 Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Equity-based compensation cost during the period with respect to the award, which will be recognized in income (as well as the total recognized tax benefit) or capitalized as part of the cost of an asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 50 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6406099&loc=d3e25284-112666 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph 64, 65 -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 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph A240 -Subparagraph g(1) -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 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + 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)(e) -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 Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + 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)(c) -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 Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -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 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 The total reduction to unamortized compensation cost for restricted shares forfeited during the period. + Details
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- Definition This element represents percentage of target achieved by employee under performance shares during the reporting period. + Details
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- Definition This element represents additional compensation cost that would be recognized if all outstanding performance shares are vested. + Details
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- Definition The amount of performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria. + Details
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- Definition The number of target performance shares that were paid out (including deposits into a deferred compensation plan) or that expired due to failure to meet performance criteria. + Details
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