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Employee Share-Based Incentive Compensation Plans (OneBeacon Performance Shares Outstanding) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 9 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Sep. 30, 2012 | Jun. 30, 2012 | Dec. 31, 2011 | Sep. 30, 2011 | Jun. 30, 2011 | Dec. 31, 2010
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Accrued incentive compensation | $ 144.7 |  | $ 187.9 |  |  |
Non-Qualified Options | One Beacon Long Term Incentive Plan [Member] |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Expected life (in years) | 5 years 6 months |  |  |  |  |
Performance Share [Member] | One Beacon Long Term Incentive Plan [Member] |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award Equity Instruments Other than Options Nonvested, Number before Forfeitures | 571,511 |  |  |  |  |
Incentive Compensation Payable before Forfeitures | 0.9 |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Assumed Forfeitures in Period | (14,288) |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Assumed Forfeitures in Period Amount | 0 |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number | 557,223 | 560,577 | 642,667 | 652,144 | 671,727 | 1,464,295
Accrued incentive compensation | 0.9 | 3.1 | 9.7 | 9.6 | 10.0 | 18.5
Performance Share [Member] | One Beacon Long Term Incentive Plan [Member] | Range of Performance Cycle from 2010 to 2012 [Member] |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award Equity Instruments Other than Options Nonvested, Number before Forfeitures | 238,658 |  |  |  |  |
Incentive Compensation Payable before Forfeitures | 0 |  |  |  |  |
Performance Share [Member] | One Beacon Long Term Incentive Plan [Member] | Range of Performance Cycle from 2011 to 2013 [Member] |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award Equity Instruments Other than Options Nonvested, Number before Forfeitures | 151,563 |  |  |  |  |
Incentive Compensation Payable before Forfeitures | 0.4 |  |  |  |  |
Performance Share [Member] | One Beacon Long Term Incentive Plan [Member] | Range Of Performance Cycle From 2012 To 2014 [Member] |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award Equity Instruments Other than Options Nonvested, Number before Forfeitures | 181,290 |  |  |  |  |
Incentive Compensation Payable before Forfeitures | $ 0.5 |  |  |  |  |

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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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Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

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- Definition Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: 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.D.2) -URI http://asc.fasb.org/extlink&oid=6793087&loc=d3e301413-122809 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2)(i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section D -Subsection 2 + Details
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- 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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Balance Type: | credit
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- Definition Carrying value as of the balance sheet date for performance shares before forfeitures. + References No definition available. + Details
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Balance Type: | credit
Period Type: | instant

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- Definition This element represents the reduction for assumed forfeitures to the number of performance shares outstanding. + References No definition available. + Details
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- Definition This element represents the reduction to accrued expense for assumed forfeitures of performance shares. + References No definition available. + Details
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- Definition This element represents the number of non-vested performance shares before forfeiture assumptions. + References No definition available. + Details
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