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Employee Share-Based Incentive Compensation Plans (Details) (USD $) |  |  | 12 Months Ended | 1 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended
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Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2011 OneBeacons employee stock ownership plan | Dec. 31, 2011 Sirius Group's profit sharing plan | Dec. 31, 2011 Performance shares Y multiplier | May 31, 2010 Non-Qualified Options | Dec. 31, 2011 Non-Qualified Options | Dec. 31, 2010 Non-Qualified Options | Dec. 31, 2009 Non-Qualified Options | Dec. 31, 2011 WTM Incentive Plan and WTM Phantom Share Plans Performance shares | Dec. 31, 2010 WTM Incentive Plan and WTM Phantom Share Plans Performance shares | Dec. 31, 2009 WTM Incentive Plan and WTM Phantom Share Plans Performance shares | May 31, 2011 OneBeacon Long-Term Incentive Plan Performance shares | Dec. 31, 2011 OneBeacon Long-Term Incentive Plan Performance shares | Dec. 31, 2010 OneBeacon Long-Term Incentive Plan Performance shares | Dec. 31, 2009 OneBeacon Long-Term Incentive Plan Performance shares | Dec. 31, 2010 OneBeacon Long-Term Incentive Plan Performance shares Performance cycle 2007-2009 | Dec. 31, 2009 OneBeacon Long-Term Incentive Plan Performance shares Performance cycle 2007-2008 | Dec. 31, 2011 OneBeacon Long-Term Incentive Plan Performance shares Performance cycle 2008-2010 | Dec. 31, 2011 OneBeacon Long-Term Incentive Plan Non-Qualified Options | Dec. 31, 2010 OneBeacon Long-Term Incentive Plan Non-Qualified Options | Dec. 31, 2009 OneBeacon Long-Term Incentive Plan Non-Qualified Options
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Vesting period (in years) |  |  |  |  |  |  |  |  |  | 3 years |  |  |  |  |  |  |  |  |  | Third, fourth and fifth anniversaries |  |
Annual base contribution in common shares provided to participants expressed as a percentage of salary (as a percent) |  |  | 3.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Social Security wage base amount, maximum |  |  | $ 106,800 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of variable contribution for participants not otherwise eligible to receive certain other OneBeacon benefits |  |  | 6.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of variable contribution for participants under Sirius Group's profit sharing plan |  |  |  | 7.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Multiplier of number of shares initially granted to determine payout, low end of range |  |  |  |  | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Multiplier of number of shares initially granted to determine payout, high end of range |  |  |  |  | 2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Performance cycle (in years) |  |  |  |  | 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Beginning of period (in shares) |  |  |  |  |  |  |  |  |  | 163,184 | 172,454 | 156,373 |  | 1,464,295 | 2,224,215 | 2,212,313 |  |  |  |  |  |
Shares expired (in shares) |  |  |  |  |  |  |  |  |  | (51,131) | (49,127) | (48,573) |  | (936,150) | (889,594) | (137,400) |  |  |  |  |  |
New grants (in shares) |  |  |  |  |  |  |  |  |  | 37,675 | 47,880 | 69,092 |  | 194,900 | 270,691 | 379,085 |  |  |  |  |  |
Cancellations and change in assumed forfeitures (in shares) |  |  |  |  |  |  |  |  |  | 336 | (8,023) | (4,438) | (35,000) | (80,378) | (141,017) | (229,783) |  |  |  |  |  |
Ending of period (in shares) |  |  |  |  |  |  |  |  |  | 150,064 | 163,184 | 172,454 |  | 642,667 | 1,464,295 | 2,224,215 |  |  |  |  |  |
Expense Amortized |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Accrued expense at the beginning of period | 187,900,000 | 157,400,000 |  |  |  |  |  |  |  | 29,400,000 | 12,200,000 | 4,200,000 |  | 18,500,000 | 15,100,000 | 4,600,000 |  |  |  | 4,500,000 | 3,600,000 | 2,500,000
Payments and deferrals |  |  |  |  |  |  |  |  |  |  |  |  |  | (10,500,000) | (4,600,000) |  |  |  |  |  |  |
Assumed forfeitures and cancellations |  |  |  |  |  |  |  |  |  | (900,000) | (700,000) | (300,000) |  | (500,000) | (2,200,000) | (500,000) |  |  |  |  |  |
Expense (income) recognized |  |  |  |  |  | 8,700,000 | 100,000 | 11,900,000 | 5,400,000 | 37,600,000 | 17,900,000 | 8,300,000 |  | 2,200,000 | 10,200,000 | 11,000,000 |  |  |  | 500,000 | 900,000 | 1,100,000
Accrued expense at the end of the period | 187,900,000 | 157,400,000 |  |  |  |  |  |  |  | 66,100,000 | 29,400,000 | 12,200,000 |  | 9,700,000 | 18,500,000 | 15,100,000 |  |  |  | 5,000,000 | 4,500,000 | 3,600,000
Percentage of target achieved by former employees as a result of the Commercial Lines and Personal Lines Transactions |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 14.20% | 1.40% | 68.50% |  |  |
Target vesting percentage of performance shares |  |  |  |  |  |  |  |  |  | 100.00% |  |  |  | 100.00% |  |  |  |  |  |  |  |
Additional compensation cost that would be recognized if all outstanding performance shares vested |  |  |  |  |  |  |  |  |  | $ 19,300,000 |  |  |  | $ 2,300,000 |  |  |  |  |  |  |  |

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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 The total reduction to unamortized compensation cost for restricted shares forfeited during the period. + References No definition available. + Details
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- Definition Represents the employer's annual contribution (as a percentage of salary up to the Social Security wage base) to the ESOP plan. + References No definition available. + Details
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- Definition Represent the amount of contributed by the employee maximum amount applicable to social security wage base. + References No definition available. + Details
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- Definition Represents the employer's maximum annual variable contribution (as a percentage of salary up to the Social Security wage base) to the ESOP plan for employees who are not otherwise eligible for certain incentive compensation benefits. + 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 This element represents percentage of target achieved by employee under performance shares during the reporting period. + References No definition available. + Details
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- Definition This element represents the target percentage vesting for the performance shares. + References No definition available. + Details
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- Definition Represents the employer's maximum annual variable contribution (as a percentage of salary) to the profit sharing plan for employees. + References No definition available. + Details
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- Definition This element represents additional compensation cost that would be recognized if all outstanding performance shares are vested. + References No definition available. + 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. + References No definition available. + 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. + References No definition available. + Details
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- Definition Represents the high end of the range of multiplier for number of shares initially granted to determine payout. + References No definition available. + Details
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- Definition Represents the low end of the range of the multiplier for the number of shares initially granted to determine payout. + References No definition available. + Details
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- Definition Represents the performance cycle on conclusion of which shares become payable. + References No definition available. + Details
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