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Employee Share-Based Incentive Compensation Plans (OneBeason Peformance Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended
--- | ---
Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Expense Amortized |  |  |  |  |  |
Accrued expense at the beginning of period | $ 187.9 |  |  |  |  |
Accrued expense at the end of the period | 159.0 |  |  |  |  |
Vesting rate for third fourth and fifth anniversary | 33.00% |  |  |  |  |
One Beacon Long Term Incentive Plan |  |  |  |  |  |
Target options outstanding |  |  |  |  |  |
Beginning of period (in shares) | 642,667 |  | 1,464,295 |  | 2,224,215 |
Shares paid or expired (in shares) | (258,901) | [1],[2] | (936,150) | [1],[2] | (889,594) | [1],[2]
New grants (in shares) | 181,290 |  | 194,900 |  | 270,691 |
Assumed forfeitures and cancellations (in shares) | (1,866) |  | (80,378) |  | (141,017) |
Ending of period (in shares) | 563,190 |  | 642,667 |  | 1,464,295 |
Expense Amortized |  |  |  |  |  |
Accrued expense at the beginning of period | 9.7 |  | 18.5 |  | 15.1 |
Payments and deferrals | (7.7) | [1],[2] | (10.5) | [1],[2] | (4.6) | [1],[2]
Assumed forfeitures and cancelations | 0 |  | (0.5) |  | (2.2) |
Expense recognized | (0.8) |  | 2.2 |  | 10.2 |
Accrued expense at the end of the period | $ 1.2 |  | $ 9.7 |  | $ 18.5 |
Performance Cycle From 2009 to 2011 | One Beacon Long Term Incentive Plan |  |  |  |  |  |
Expense Amortized |  |  |  |  |  |
Percentage of payout for minimum growth target percentage | 138.60% |  |  |  |  |
One Beacon Long Term Incentive Plan | Performance Share |  |  |  |  |  |
Expense Amortized |  |  |  |  |  |
Target vesting percentage | 100.00% |  |  |  |  |
Range of Performance Cycle from 2008 to 2010 | One Beacon Long Term Incentive Plan |  |  |  |  |  |
Expense Amortized |  |  |  |  |  |
Percentage of payout for minimum growth target percentage | 68.50% |  |  |  |  |
Range of Performance Cycle from 2007 to 2009 | One Beacon Long Term Incentive Plan |  |  |  |  |  |
Expense Amortized |  |  |  |  |  |
Percentage of payout for minimum growth target percentage | 14.20% |  |  |  |  |
[1] | OneBeacon performance share payments in 2010 also include accelerated payments resulting from the OneBeacon Personal Lines and Commercial Lines Transactions to employees of those businesses. The accelerated OneBeacon performance shares payments for the 2009-2011 and 2010-2012 performance cycles were on a pro rata basis and at a performance factor of 100%.
[2] | OneBeacon performance share payments in 2012 for the 2009-2011 performance cycle were at 138.6% of target. OneBeacon performance shares payments in 2011 for the 2008-2010 performance cycle were at 68.5% of target. OneBeacon performance share payments in 2010 for the 2007-2009 performance cycle were at 14.2% of target. Amounts include deposits into OneBeacon’s deferred compensation plan.

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + 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 (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 2: 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 3: 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 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 123R -Paragraph 64 -Subparagraph 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 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 14.F) -URI http://asc.fasb.org/extlink&oid=6793087&loc=d3e301413-122809 + 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 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 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)(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. + 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 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 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)(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. + 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 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 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. + Details
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- Definition The total reduction to unamortized compensation cost for performance shares forfeited during the period. + 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 the target percentage vesting for the performance shares. + 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 lpercentage of target achieved, low end of range. + References No definition available. + Details
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- Definition Vesting Rate, Third, Fourth and Fifth Anniversary + References No definition available. + Details
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