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Third Party Reinsurance (One Beacon) (Details) - USD ($) $ in Millions | 4 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Sep. 30, 2015 | Sep. 30, 2015 | Dec. 31, 2014
Reinsurance Retention Policy [Line Items] |  |  |
Reinsurance recoverable on paid losses | $ 22.6 | $ 22.6 | $ 12.2
Reinsurance recoverable on unpaid losses | 214.6 | 214.6 | 161.7
OneBeacon |  |  |
Reinsurance Retention Policy [Line Items] |  |  |
Reinsurance recoverable on paid losses | 22.6 | 22.6 | 12.2
Reinsurance recoverable on unpaid losses | 214.2 | $ 214.2 | $ 161.6
Crop insurance [Member] | OneBeacon |  |  |
Reinsurance Retention Policy [Line Items] |  |  |
Percentage of par value of policy reinsured |  | 52.00% |
Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage |  | 98.00% |
Percentage of Quota Share Reinsurance Agreement |  | 30.00% |
Crop-hail insurance [Member] | OneBeacon |  |  |
Reinsurance Retention Policy [Line Items] |  |  |
Percentage of par value of policy reinsured |  | 80.00% |
Reinsurance Retention Policy, Loss Ratio Reinsured risk, Percentage |  | 100.00% |
Crop-hail and multiple peril crop insurance [Member] | OneBeacon |  |  |
Reinsurance Retention Policy [Line Items] |  |  |
Percentage of par value of policy reinsured |  | 100.00% |
OneBeacon |  |  |
Reinsurance Retention Policy [Line Items] |  |  |
Reinsurance Retention Policy, Excess Retention, Amount Reinsured |  | $ 130.0 |
Reinsurance Retention Policy, Amount Retained |  | $ 20.0 |
OneBeacon | Combination One |  |  |
Reinsurance Retention Policy [Line Items] |  |  |
Percentage of par value of policy reinsured |  | 95.00% |
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation |  | $ 10.0 |
OneBeacon | Minimum | Property [Member] |  |  |
Reinsurance Retention Policy [Line Items] |  |  |
Reinsurance Retention Policy, Amount Retained |  | 3.0 |
OneBeacon | Minimum | Health Care [Member] |  |  |
Reinsurance Retention Policy [Line Items] |  |  |
Reinsurance Retention Policy, Amount Retained | 3.0 |  |
OneBeacon | Maximum | Property [Member] |  |  |
Reinsurance Retention Policy [Line Items] |  |  |
Reinsurance Retention Policy, Amount Retained |  | $ 5.0 |
OneBeacon | Maximum | Health Care [Member] |  |  |
Reinsurance Retention Policy [Line Items] |  |  |
Reinsurance Retention Policy, Amount Retained | $ 5.0 |  |
OneBeacon | Combination Two |  |  |
Reinsurance Retention Policy [Line Items] |  |  |
Percentage of par value of policy reinsured |  | 100.00% |
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation |  | $ 100.0 |

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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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