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Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Shares) (Details) (OneBeacon, One Beacon Long Term Incentive Plan, USD $) In Millions, except Share data, unless otherwise specified | 0 Months Ended | 3 Months Ended | 9 Months Ended |  | 1 Months Ended |  | 3 Months Ended | 9 Months Ended | 1 Months Ended
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Mar. 01, 2012 Restricted Stock | Sep. 30, 2014 Restricted Stock | Sep. 30, 2013 Restricted Stock | Sep. 30, 2014 Restricted Stock | Sep. 30, 2013 Restricted Stock | Feb. 28, 2012 Restricted Stock Installment | May 31, 2011 Restricted Stock Chief Executive Officer | May 25, 2011 Restricted Stock Chief Executive Officer Installment | Sep. 30, 2014 Performance Share | Sep. 30, 2013 Performance Share | Sep. 30, 2014 Performance Share | Sep. 30, 2013 Performance Share | May 31, 2011 Performance Share Chief Executive Officer | May 31, 2011 Performance Cycle 2011 to 2013 Performance Share Chief Executive Officer
Target Restricted Share Activity |  |  |  |  |  |  |  |  |  |  |  |  |  |
Beginning of period (in shares) |  | 612,500 | 918,000 | 915,000 | 927,000 |  |  |  | 512,938 | 489,867 | 493,421 | 563,190 |  |
Issued (in shares) | 300,000 | 0 | 0 | 0 | 0 |  | 630,000 |  |  |  |  |  |  |
Vested (in shares) |  | 0 | 0 | (300,000) | (9,000) |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |
Assumed forfeitures and cancellations |  | 0 | (3,000) | (2,500) | (3,000) |  |  |  | 0 | 0 | (4,145) | (13,665) |  |
Ending of period (in shares) |  | 612,500 | 915,000 | 612,500 | 915,000 |  |  |  | 512,938 | 489,867 | 512,938 | 489,867 |  |
Unamortized grant date fair value |  |  |  |  |  |  |  |  |  |  |  |  |  |
Unamortized grant date fair value, beginning balance |  | 4.9 | 8.0 | 6.5 | 9.6 |  |  |  |  |  |  |  |  |
Issued |  | 0 | 0 | 0 | 0 |  |  |  |  |  |  |  |  |
Assumed forfeitures and cancellations |  | 0 | 0 | 0 | 0 |  |  |  | 0 | 0 | 0 | (0.1) |  |
Expense recognized |  | (0.7) | (0.7) | (2.3) | (2.3) |  |  |  | 1.1 | 0.1 | 2.5 | 1.4 |  |
Unamortized grant date fair value, ending balance |  | $ 4.2 | $ 7.3 | $ 4.2 | $ 7.3 |  |  |  | $ 3.7 |  | $ 3.7 |  |  |
Number of Installments in which the Shares Vest |  |  |  |  |  | 2 |  | 4 |  |  |  |  |  |
Annual reduction in performance shares |  |  |  |  |  |  |  |  |  |  |  |  |  | 35,000
Years Performance Shares will be Forfeited |  |  |  |  |  |  |  |  |  |  |  |  | 5 years |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Reduction in the next five years |  |  |  |  |  |  | 35,000 |  |  |  |  |  |  |

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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 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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