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Third-Party Reinsurance (OneBeacon) (Details) (USD $) In Millions, unless otherwise specified |  |  |  |  | 12 Months Ended |  | 12 Months Ended |
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Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2013 OneBeacon | Dec. 31, 2013 OneBeacon | Dec. 31, 2013 Casualty Reinsurance Program OneBeacon | Dec. 31, 2013 Property Per-Risk, Acts of Terrorism OneBeacon | Dec. 31, 2012 Property Per-Risk, Acts of Terrorism OneBeacon | Dec. 31, 2013 Property Per-Risk, Acts of Terrorism Maximum OneBeacon | Dec. 31, 2013 Property Per-Risk, Acts of Terrorism OneBeacon Maximum | Dec. 31, 2013 Property Per-Risk, Acts of Terrorism OneBeacon Minimum | Dec. 31, 2013 Property Catastrophe Reinsurance OneBeacon | Dec. 31, 2013 Property Catastrophe Reinsurance Minimum OneBeacon | Dec. 31, 2013 Property Catastrophe Reinsurance OneBeacon | Dec. 31, 2013 Property Catastrophe Reinsurance OneBeacon Maximum | Dec. 31, 2012 Property Catastrophe Reinsurance OneBeacon Maximum | Dec. 31, 2013 Property Per Risk Reinsurance Program OneBeacon | Dec. 31, 2013 Property Per Risk Reinsurance Program OneBeacon Maximum | Dec. 31, 2013 Property Per Risk Reinsurance Program OneBeacon Minimum | Dec. 31, 2013 Layer One Property Catastrophe Reinsurance OneBeacon | Dec. 31, 2013 Layer One Property Catastrophe Reinsurance Minimum OneBeacon | Dec. 31, 2013 Layer One Property Catastrophe Reinsurance OneBeacon Maximum | Dec. 31, 2013 Layer One Property Per Risk Reinsurance Program OneBeacon Maximum | Dec. 31, 2013 Layer One Property Per Risk Reinsurance Program OneBeacon Minimum | Dec. 31, 2013 Layer Two Property Catastrophe Reinsurance OneBeacon | Dec. 31, 2013 Layer Two Property Catastrophe Reinsurance OneBeacon Maximum | Dec. 31, 2013 Layer Two Property Catastrophe Reinsurance OneBeacon Minimum | Dec. 31, 2013 Layer Two Property Per Risk Reinsurance Program OneBeacon | Dec. 31, 2013 Layer Two Property Per Risk Reinsurance Program OneBeacon Minimum | Dec. 31, 2013 Layer Three Property Catastrophe Reinsurance OneBeacon | Dec. 31, 2013 Layer Three Property Catastrophe Reinsurance OneBeacon Maximum | Dec. 31, 2013 Layer Three Property Catastrophe Reinsurance OneBeacon Minimum | Dec. 31, 2013 Inland and Ocean Marine Marine Treaty Per Occurrence OneBeacon | Dec. 31, 2013 Inland and Ocean Marine Marine Treaty OneBeacon | Dec. 31, 2013 Inland and Ocean Marine Layer One Marine Treaty OneBeacon | Dec. 31, 2013 Inland and Ocean Marine Layer One Marine Treaty OneBeacon Maximum | Dec. 31, 2013 Inland and Ocean Marine Layer One Marine Treaty OneBeacon Minimum | Dec. 31, 2013 Inland and Ocean Marine Layer One Property Per Risk Treaty OneBeacon Maximum | Dec. 31, 2013 Inland and Ocean Marine Layer One Catastrophic Losses [Member] OneBeacon | Dec. 31, 2013 Catastrophic Losses [Member] Layer One OneBeacon | Dec. 31, 2013 Surety Underwriting Insurance Per Bond OneBeacon | Dec. 31, 2013 Surety Underwriting Insurance Per Bond OneBeacon Maximum | Dec. 31, 2013 Surety Underwriting Insurance Per Bond OneBeacon Minimum | Dec. 31, 2013 Health Insurance Product Line Casualty Reinsurance Program OneBeacon Maximum | Dec. 31, 2013 Health Insurance Product Line Casualty Reinsurance Program OneBeacon Minimum | Dec. 31, 2013 Health Insurance Product Line Layer One Casualty Reinsurance Program OneBeacon | Dec. 31, 2013 Property, Liability and Casualty Insurance Product Line Casualty Reinsurance Program OneBeacon Maximum | Dec. 31, 2013 Property, Liability and Casualty Insurance Product Line Casualty Reinsurance Program OneBeacon Minimum | Dec. 31, 2013 Property, Liability and Casualty Insurance Product Line Layer One Casualty Reinsurance Program OneBeacon | Dec. 31, 2013 Property, Liability and Casualty Insurance Product Line Layer Two Casualty Reinsurance Program | Dec. 31, 2013 Property, Liability and Casualty Insurance Product Line Layer Two Casualty Reinsurance Program OneBeacon | Dec. 31, 2013 Workers compensation Casualty Reinsurance Program OneBeacon | Dec. 31, 2013 Workers compensation Casualty Reinsurance Program OneBeacon Maximum | Dec. 31, 2013 Workers compensation Casualty Reinsurance Program OneBeacon Minimum | Dec. 31, 2013 AA OneBeacon | Dec. 31, 2013 A OneBeacon | Dec. 31, 2013 BBB OneBeacon
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverable on paid and unpaid losses |  |  |  |  | $ 89.9 | [1] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 26.4 | [1] | $ 56.1 | [1] | $ 7.4 | [1]
Reinsurance recoverables, percentage of total |  |  |  |  | 100.00% | [1] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 30.00% | [1] | 62.00% | [1] | 8.00% | [1]
Percentage of certified losses required to be shared by federal government subject to terrorism risk insurance act |  |  |  |  | 85.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  |  |  |  |  | 10.0 | 100.0 |  | 100,000.0 |  |  |  | 20.0 | 33.0 | 150.0 | 180.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.0 |  |  |  |  | 20.0 | 2.0 |  |  |  |  |  |  | 5.0 |  |  | 5.0 |  | 11.0 |  |  |  |  |  |  |  |  |
Annual Aggregate Deductible to Reinsurance Retention Policy |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1.5 | 5.0 |  |  |  |  |  |  |  |  |  | 3.0 |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation |  |  |  |  |  |  |  |  |  |  |  |  |  | 117.0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  |  |  |  |  | 60.0 |  |  |  | 100.0 | 10.0 | 130.0 |  |  |  |  |  | 100.0 | 10.0 |  | 20.0 | 30.0 | 40.0 | 20.0 |  | 70.0 | 30.0 |  | 40.0 |  | 150.0 | 70.0 |  | 60.0 |  | 7.0 | 2.0 |  | 5.0 |  | 60.0 | 30.0 | 5.0 | 20.0 | 5.0 | 5.0 | 21.0 | 5.0 | 6.0 |  | 10.0 |  | 10.0 | 1.0 |  |  |  |  |  |
Property Catastrophe Reinsurance Treaty Second Layer Co-Partipation Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 10.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Property Catastrophe Reinsurance Treaty First Layer Co-Partipation Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 60.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Excess Retention, Percentage |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 50.00% |  |  |  |  | 10.00% |  |  |  |  | 5.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage |  |  |  |  |  |  |  |  |  | 85.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 100.00% |  |  |  |  | 10.00% |  |  | 10.00% |  | 5.00% | 100.00% |  |  |  |  |  |  |  |
Reinsurance recoverable on paid losses |  |  |  |  |  |  | 9.7 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reinsurance recoverable on unpaid losses | $ 428.1 | $ 429.1 | $ 2,507.3 | $ 2,344.0 |  |  | $ 80.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | Standard & Poor’s ratings as detailed above are: “AA” (Very strong), “A” (Strong) and “BBB+” (Adequate).

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- Definition Estimated amount after valuation allowance of reinsurance recoverables, due from reinsurers for incurred claims not yet reported, and claims settlement expenses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21409-158489 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 7 + Details
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- Definition Amount after valuation allowance currently billed or billable under reinsurance contracts. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21409-158489 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.6) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 7 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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- Definition Amount after valuation allowance recoverable including premium paid under reinsurance contracts. Examples include, but are not limited to, settled and unsettled claims, incurred but not reported losses, loss adjustment expense, premium paid, policy benefits and policy reserves. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6824-158387 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.(a),6) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
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- Definition Stated amount that is the upper retention limit per entity policy. + References No definition available. + Details
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- Definition Percentage of risk reinsured in excess of stated retention per entity policy. + References No definition available. + Details
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