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Third Party Reinsurance (One Beacon) (Details) (USD $) In Millions, unless otherwise specified |  |  |  |  | 9 Months Ended | 2 Months Ended | 4 Months Ended | 9 Months Ended
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Sep. 30, 2014 | Dec. 31, 2013 | Sep. 30, 2014 OneBeacon | Dec. 31, 2013 OneBeacon | Sep. 30, 2014 Crop insurance OneBeacon | Sep. 30, 2014 Crop-hail insurance OneBeacon | Sep. 30, 2014 Property Catastrophe Reinsurance [Member] Property [Member] OneBeacon | Jun. 30, 2014 Property Per Risk Reinsurance Program [Member] OneBeacon | Apr. 30, 2014 Property Per Risk Reinsurance Program [Member] OneBeacon | Sep. 30, 2014 Minimum Property Catastrophe Reinsurance [Member] Property [Member] OneBeacon | Sep. 30, 2014 Maximum Property Catastrophe Reinsurance [Member] Property [Member] OneBeacon
Reinsurance Retention Policy [Line Items] |  |  |  |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Retained Percentage |  |  |  |  | 101.50% | 100.00% |  |  |  |  |
Reinsurance recoverable on paid losses | $ 27.2 | $ 25.4 | $ 12.7 | $ 9.7 |  |  |  |  |  |  |
Reinsurance recoverable on unpaid losses | 430.7 | 428.1 | 101.4 | 80.2 |  |  |  |  |  |  |
Reinsurance Retention Policy, Amount Retained |  |  |  |  |  |  |  | 5.0 | 10.0 | 20.0 | 130.0
Percentage of par value of policy reinsured |  |  |  |  | 48.50% | 80.00% | 100.00% |  |  |  |
Reinsurance Retention Policy, Excess Retention, Amount Reinsured |  |  |  |  |  |  | $ 110.0 |  |  |  |

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- Definition Amount after valuation allowance of reinsurance recoverables on paid losses. + 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 + Details
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- Definition Amount after valuation allowance of reinsurance recoverables for losses reported to the ceding insurer but not yet paid, amounts expected of incurred losses and settlement expenses, which have not yet been reported to the ceding insurer. + 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 + 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 Amount of risk reinsured in excess of stated retention per company policy. + References No definition available. + Details
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- Details
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- Definition Percentage of risk reinsured on insurance products per entity policy. + References No definition available. + Details
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- Definition Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage + References No definition available. + Details
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