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Third Party Reinsurance (One Beacon) (Details) (USD $) In Millions, unless otherwise specified |  |  |  |  | 3 Months Ended
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Mar. 31, 2014 | Dec. 31, 2013 | Mar. 31, 2014 OneBeacon | Dec. 31, 2013 OneBeacon | Mar. 31, 2014 Crop insurance [Member] OneBeacon | Mar. 31, 2014 Crop-hail insurance [Member] OneBeacon
Reinsurance Retention Policy [Line Items] |  |  |  |  |  |
Reinsurance Retention Policy, Retained Percentage |  |  |  |  | 101.50% | 100.00%
Reinsurance recoverable on paid losses | $ 15.2 | $ 25.4 | $ 3.0 | $ 9.7 |  |
Reinsurance recoverable on unpaid losses | $ 425.2 | $ 428.1 | $ 80.1 | $ 80.2 |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage |  |  |  |  | 48.50% | 80.00%

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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 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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