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Third Party Reinsurance (Recoverable Amounts by Reinsurer) (Details) - USD ($) $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance recoverable on unpaid losses | $ 172.9 | $ 186.0 | $ 161.6 | $ 80.2
Reinsurance recoverable on paid and unpaid losses | 179.5 | 193.5 |  |
OneBeacon |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance recoverable on paid and unpaid losses | 179.5 | $ 193.5 |  |
OneBeacon | Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance Recoverables | $ 179.5 |  |  |
Reinsurance recoverables, percentage of total | 100.00% |  |  |
Minimum | OneBeacon | Property Catastrophe Program |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, amount retained | $ 20.0 |  |  |
Maximum | OneBeacon | Property Catastrophe Program |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 130.0 |  |  |
OneBeacon |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance recoverable on unpaid losses | 172.9 |  |  |
OneBeacon | Specialty Property |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 34.0 |  |  |
Reinsurance retention policy, amount retained | 6.0 |  |  |
OneBeacon | Combined Healthcare [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, amount retained | 10.0 |  |  |
OneBeacon | Surety Reinsurance Program [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 45.0 |  |  |
Reinsurance retention policy, amount retained | 5.0 |  |  |
OneBeacon | Property Catastrophe Program |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 110.0 |  |  |
Reinsurance retention policy, amount retained | 20.0 |  |  |
OneBeacon | Property Per Risk Reinsurance Program |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 100.0 |  |  |
Reinsurance retention policy, amount retained | 3.0 |  |  |
OneBeacon | HMO Provider Excess Reinsurance [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, amount retained | 5.0 |  |  |
OneBeacon | Inland and Ocean Marine |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 57.5 |  |  |
Reinsurance retention policy, amount retained | 2.5 |  |  |
OneBeacon | Film Completion Bond Business [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 38.0 |  |  |
Reinsurance retention policy, amount retained | 2.0 |  |  |
OneBeacon | Casualty Clash/ Workers Compensation Catastrophe [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 34.0 |  |  |
Reinsurance retention policy, amount retained | 6.0 |  |  |
OneBeacon | Workers compensation |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 20.0 |  |  |
Reinsurance retention policy, amount retained | 40.0 |  |  |
OneBeacon | Financial Institutions - Professional Liability Reinsurance [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 10.0 |  |  |
OneBeacon | Casualty 2nd layer |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, amount retained | 11.0 |  |  |
OneBeacon | Other Casualty Business [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 8.0 |  |  |
Reinsurance retention policy, amount retained | 3.0 |  |  |
OneBeacon | Workers compensation per occurrence [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 8.0 |  |  |
Reinsurance retention policy, amount retained | 2.0 |  |  |
OneBeacon | Healthcare Professional Liability [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 7.0 |  |  |
Reinsurance retention policy, amount retained | 3.0 |  |  |
OneBeacon | Maximum | Specialty Property |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 6.0 |  |  |
OneBeacon | Maximum | Surety Reinsurance Program [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 5.0 |  |  |
OneBeacon | Maximum | Property Catastrophe Program |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 20.0 |  |  |
OneBeacon | Maximum | Property Per Risk Reinsurance Program |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 3.0 |  |  |
OneBeacon | Maximum | HMO Provider Excess Reinsurance [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 5.0 |  |  |
OneBeacon | Maximum | Inland and Ocean Marine |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 7.0 |  |  |
OneBeacon | Maximum | Film Completion Bond Business [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 2.0 |  |  |
OneBeacon | Maximum | Casualty Clash/ Workers Compensation Catastrophe [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 6.0 |  |  |
OneBeacon | Maximum | Workers compensation |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 6.0 |  |  |
OneBeacon | Maximum | Financial Institutions - Professional Liability Reinsurance [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 5.0 |  |  |
OneBeacon | Maximum | Other Casualty Business [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 3.0 |  |  |
OneBeacon | Maximum | Workers compensation per occurrence [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 2.0 |  |  |
OneBeacon | Maximum | Healthcare Professional Liability [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured | 3.0 |  |  |
One Beacon Insurance Company |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance recoverable on unpaid losses | 18.3 |  |  |
AM Best A plus | OneBeacon | Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance Recoverables | $ 73.9 |  |  |
Reinsurance recoverables, percentage of total | 41.00% |  |  |
AM Best A minus | OneBeacon | Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance Recoverables | $ 79.7 |  |  |
Reinsurance recoverables, percentage of total | 44.00% |  |  |
AM Best B | OneBeacon | Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] |  |  |  |
Reinsurance Retention Policy [Line Items] |  |  |  |
Reinsurance Recoverables | $ 25.9 |  |  |
Reinsurance recoverables, percentage of total | 15.00% |  |  |

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