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Third-Party Reinsurance (OneBeacon) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended | 60 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | ---
Mar. 31, 2017 | Dec. 31, 2020 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2020 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance recoverable on paid losses |  |  |  | $ 6.6 |  | $ 7.5 |  |
Reinsurance recoverable on unpaid losses |  |  |  | 172.9 |  | $ 186.0 | $ 161.6 | $ 80.2
OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance recoverable on paid losses |  |  |  | 6.6 |  |  |  |
Reinsurance recoverable on unpaid losses |  |  |  | 172.9 |  |  |  |
Property Per Risk Reinsurance Program | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  | 3.0 |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 100.0 |  |  |  |
Property Per Risk Reinsurance Program | OneBeacon | Maximum |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 3.0 |  |  |  |
Workers compensation | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  | 40.0 |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 20.0 |  |  |  |
Workers compensation | OneBeacon | Maximum |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | $ 6.0 |  |  |  |
Property Catastrophe Program | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage |  |  |  | 100.00% |  |  |  |
Reinsurance Retention Policy Excess Retention Amount Reinsured Net Of Participation |  |  |  | $ 110.0 |  |  |  |
Property Catastrophe Program | Maximum | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 130.0 |  |  |  |
Property Catastrophe Program | Minimum | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  | 20.0 |  |  |  |
Property Catastrophe Program | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  | 20.0 |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 110.0 |  |  |  |
Property Catastrophe Program | OneBeacon | Maximum |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 20.0 |  |  |  |
Specialty Property | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  | 6.0 |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 34.0 |  |  |  |
Specialty Property | OneBeacon | Maximum |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 6.0 |  |  |  |
Film Completion Bond Business [Member] | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  | 2.0 |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 38.0 |  |  |  |
Film Completion Bond Business [Member] | OneBeacon | Maximum |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 2.0 |  |  |  |
Combined Healthcare [Member] | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  | 10.0 |  |  |  |
Inland and Ocean Marine | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  | 2.5 |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 57.5 |  |  |  |
Inland and Ocean Marine | OneBeacon | Maximum |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 7.0 |  |  |  |
Combined Healthcare/ Casualty | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 10.0 |  |  |  |
Combined Healthcare/ Casualty | OneBeacon | Maximum |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 3.0 |  |  |  |
Property Per-Risk, Acts of Terrorism | Maximum | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 100,000.0 |  |  |  |
Surety Reinsurance Program [Member] | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  | 5.0 |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 45.0 |  |  |  |
Surety Reinsurance Program [Member] | OneBeacon | Maximum |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 5.0 |  |  |  |
Other Casualty Business [Member] | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  | 3.0 |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 8.0 |  |  |  |
Other Casualty Business [Member] | OneBeacon | Maximum |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 3.0 |  |  |  |
HMO Provider Excess Reinsurance [Member] | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  | 5.0 |  |  |  |
HMO Provider Excess Reinsurance [Member] | OneBeacon | Maximum |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 5.0 |  |  |  |
Healthcare Professional Liability [Member] | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  | 3.0 |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 7.0 |  |  |  |
Healthcare Professional Liability [Member] | OneBeacon | Maximum |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  |  |  | 3.0 |  |  |  |
Casualty 2nd layer | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  |  | 11.0 |  |  |  |
Scenario, Forecast [Member] | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Percentage of certified losses required to be shared by federal government subject to terrorism risk insurance act |  | 80.00% | 83.00% |  |  |  |  |
Decreasing Annual Percentage of Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act |  |  |  |  | 1.00% |  |  |
Retained Percentage Relating To Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act |  | 20.00% | 17.00% |  |  |  |  |
Increased Annual Retained Percentage Relating to Certified Losses Required to be Shared by Federal Government Subject to Terrorism Risk Insurance Act | 1.00% |  |  |  |  |  |  |
Scenario, Forecast [Member] | Property Per-Risk, Acts of Terrorism | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  | $ 140.0 |  |  |  |  |
Reinsurance Retention Policy, Reinsured Risk, Percentage |  |  | 83.00% |  |  |  |  |
Scenario, Forecast [Member] | Property Per-Risk, Acts of Terrorism | Maximum | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, amount retained |  |  | $ 100,000.0 |  |  |  |  |
Department of Treasury [Member] | Scenario, Forecast [Member] | Property and Casualty, Commercial Insurance Product Line [Member] | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance retention policy, excess retention, amount reinsured |  | $ 200.0 | $ 140.0 |  | $ 20.0 |  |  |
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance Recoverables |  |  |  | $ 179.5 |  |  |  |
Reinsurance recoverables, percentage of total |  |  |  | 100.00% |  |  |  |
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | AM Best A plus | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance Recoverables |  |  |  | $ 73.9 |  |  |  |
Reinsurance recoverables, percentage of total |  |  |  | 41.00% |  |  |  |
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | AM Best A minus | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance Recoverables |  |  |  | $ 79.7 |  |  |  |
Reinsurance recoverables, percentage of total |  |  |  | 44.00% |  |  |  |
Reinsurer Concentration Risk [Member] | Reinsurance Recoverable [Member] | AM Best B | OneBeacon |  |  |  |  |  |  |  |
Effects of Reinsurance [Line Items] |  |  |  |  |  |  |  |
Reinsurance Recoverables |  |  |  | $ 25.9 |  |  |  |
Reinsurance recoverables, percentage of total |  |  |  | 15.00% |  |  |  |

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