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Employee Share-Based Incentive Compensation Plans (OneBeacon Restsricted Shares) (Details) (USD $) In Millions, except Share data, unless otherwise specified | 12 Months Ended | 1 Months Ended | 12 Months Ended | 0 Months Ended | 1 Months Ended |
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Dec. 31, 2011 | Dec. 31, 2013 Restricted Stock | May 31, 2011 Restricted Stock One Beacon Insurance Company Long Term Incentive Plan | Dec. 31, 2013 Restricted Stock One Beacon Insurance Company Long Term Incentive Plan | Dec. 31, 2012 Restricted Stock One Beacon Insurance Company Long Term Incentive Plan | Dec. 31, 2011 Restricted Stock One Beacon Insurance Company Long Term Incentive Plan | Mar. 01, 2012 Restricted Stock One Beacon Insurance Company Long Term Incentive Plan Chief Executive Officer Installment | May 31, 2011 Restricted Stock One Beacon Insurance Company Long Term Incentive Plan Chief Executive Officer | May 25, 2011 Restricted Stock One Beacon Insurance Company Long Term Incentive Plan Chief Executive Officer Installment
Target Performance Share activity |  |  |  |  |  |  |  |  |
Beginning of period (in shares) |  |  |  | 927,000 | 630,000 | 0 |  |  |
Issued (in shares) |  |  |  | 0 | 300,000 | 630,000 | 300,000 | 630,000 |
Vested (in shares) |  |  |  | (9,000) | (667) | 0 |  |  |
Assumed forfeitures and cancellations (in shares) | 0 |  |  | (3,000) | (2,333) |  |  |  |
Ending of period (in shares) |  |  |  | 915,000 | 927,000 | 630,000 |  |  |
Beginning balance |  |  |  | $ 9.6 | $ 7.7 | $ 0 |  |  |
Issued |  |  |  | 0 | 4.5 | 8.6 |  |  |
Vested |  | 0 |  |  |  | 0 |  |  |
Assumed forfeitures and cancellations |  |  |  | 0 | 0 | 0 |  |  |
Expense recognized |  |  |  | (3.1) | (2.6) | (0.9) |  |  |
Ending balance |  |  |  | $ 6.5 | $ 9.6 | $ 7.7 |  |  |
Number of installments in which the shares vest |  |  |  |  |  |  | 2 |  | 4
Forfeited shares |  |  | 35,000 |  |  |  |  | 35,000 |
Years performance shares will be forfeited |  |  |  |  |  |  |  | 5 years |

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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 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5047-113901 Reference 2: 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=27013229&loc=d3e301413-122809 Reference 3: 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 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 14 -Section F + Details
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- Definition Unrecognized cost of unvested share-based compensation awards. + 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 (i) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + 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 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 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 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested 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)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash. + 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 (d)(2) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition Number of share instruments newly issued under a share-based compensation 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 -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 + Details
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- Definition The total fair value (on the date of issue) for restricted share awards issued during the period. + References No definition available. + Details
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