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Third Party Reinsurance (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended |
--- | --- | --- | ---
Mar. 31, 2016 | Sep. 30, 2016 | Dec. 31, 2015
Reinsurance Retention Policy [Line Items] |  |  |
Reinsurance recoverable on paid losses |  | $ 16.4 | $ 7.5
Reinsurance recoverable on unpaid losses |  | 169.4 | 186.0
OneBeacon [Member] |  |  |
Reinsurance Retention Policy [Line Items] |  |  |
Reinsurance recoverable on paid losses |  | 16.4 | 7.5
Reinsurance recoverable on unpaid losses |  | $ 169.4 | $ 186.0
OneBeacon [Member] |  |  |
Reinsurance Retention Policy [Line Items] |  |  |
Percentage of Quota Share Reinsurance Agreement | 50.00% |  |
Property Catastrophe Program [Member] | OneBeacon [Member] |  |  |
Reinsurance Retention Policy [Line Items] |  |  |
Percentage of par value of policy reinsured |  | 100.00% |
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation |  | $ 110.0 |
Property Catastrophe Program [Member] | Minimum | OneBeacon [Member] |  |  |
Reinsurance Retention Policy [Line Items] |  |  |
Reinsurance Retention Policy, Amount Retained |  | 20.0 |
Property Catastrophe Program [Member] | Maximum [Member] | OneBeacon [Member] |  |  |
Reinsurance Retention Policy [Line Items] |  |  |
Reinsurance Retention Policy, Excess Retention, Amount Reinsured |  | $ 130.0 |

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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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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Represents the percentage ceded under quota share reinsurance agreement. + References No definition available. + Details
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- Definition Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation + References No definition available. + Details
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