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Loss and Loss Adjustment Expense Reserves - Cumulative Paid Loss and Allocated LAE, Net of Reinsurance (Details) - USD ($) $ in Millions | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
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Property and Accident & Health |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | $ 337.1 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2012, net of reinsurance | 0.6 |  |  |  |  |  |  |  |  |
Loss and LAE reserves, net of reinsurance | 175.0 |  |  |  |  |  |  |  |  |
Property and Accident & Health | 2012 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 59.5 | $ 59.4 | $ 59.3 | $ 58.7 | $ 58.6 | $ 58.2 | $ 57.3 | $ 53.6 | $ 45.0 | $ 14.2
Property and Accident & Health | 2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 61.2 | 61.3 | 61.3 | 61.3 | 61.1 | 61.1 | 60.0 | 40.3 | 15.8 |
Property and Accident & Health | 2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 28.3 | 28.4 | 28.2 | 28.1 | 28.0 | 27.6 | 25.4 | 13.9 |  |
Property and Accident & Health | 2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 15.3 | 15.1 | 14.8 | 14.9 | 13.7 | 12.4 | 7.0 |  |  |
Property and Accident & Health | 2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 17.5 | 17.2 | 17.1 | 16.8 | 13.4 | 8.6 |  |  |  |
Property and Accident & Health | 2017 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 30.2 | 33.3 | 32.1 | 26.3 | 17.0 |  |  |  |  |
Property and Accident & Health | 2018 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 40.7 | 40.7 | 32.8 | 15.8 |  |  |  |  |  |
Property and Accident & Health | 2019 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 19.1 | 17.1 | 6.9 |  |  |  |  |  |  |
Property and Accident & Health | 2020 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 34.5 | 11.4 |  |  |  |  |  |  |  |
Property and Accident & Health | 2021 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 30.8 |  |  |  |  |  |  |  |  |
Marine & Energy |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 161.7 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2012, net of reinsurance | 1.2 |  |  |  |  |  |  |  |  |
Loss and LAE reserves, net of reinsurance | 99.3 |  |  |  |  |  |  |  |  |
Marine & Energy | 2012 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 31.0 | 31.3 | 31.4 | 31.7 | 31.3 | 30.6 | 30.2 | 27.4 | 24.0 | 8.1
Marine & Energy | 2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 29.9 | 29.7 | 29.9 | 29.9 | 29.7 | 29.1 | 28.1 | 22.6 | 7.9 |
Marine & Energy | 2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 13.8 | 14.0 | 13.7 | 14.5 | 14.4 | 13.5 | 12.4 | 6.0 |  |
Marine & Energy | 2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 11.2 | 10.8 | 10.7 | 11.3 | 9.8 | 8.0 | 4.0 |  |  |
Marine & Energy | 2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 14.0 | 13.4 | 13.3 | 12.8 | 10.1 | 5.6 |  |  |  |
Marine & Energy | 2017 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 14.4 | 14.4 | 13.1 | 11.3 | 5.2 |  |  |  |  |
Marine & Energy | 2018 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 15.2 | 14.5 | 12.9 | 2.7 |  |  |  |  |  |
Marine & Energy | 2019 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 12.9 | 10.9 | 3.4 |  |  |  |  |  |  |
Marine & Energy | 2020 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 12.9 | 3.2 |  |  |  |  |  |  |  |
Marine & Energy | 2021 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 6.4 |  |  |  |  |  |  |  |  |
Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 145.3 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2012, net of reinsurance | (2.0) |  |  |  |  |  |  |  |  |
Loss and LAE reserves, net of reinsurance | 85.2 |  |  |  |  |  |  |  |  |
Specialty | 2012 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 24.5 | 24.4 | 24.4 | 24.2 | 23.3 | 23.2 | 22.6 | 22.2 | 25.2 | 16.3
Specialty | 2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 16.0 | 16.0 | 16.0 | 16.0 | 15.8 | 15.8 | 15.2 | 13.6 | 17.1 |
Specialty | 2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 43.5 | 43.6 | 42.8 | 41.5 | 40.8 | 40.4 | 39.7 | 26.8 |  |
Specialty | 2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 8.3 | 8.3 | 8.2 | 8.2 | 7.8 | 7.2 | 4.1 |  |  |
Specialty | 2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 10.5 | 10.4 | 10.0 | 9.2 | 8.0 | 3.2 |  |  |  |
Specialty | 2017 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 8.5 | 8.5 | 8.4 | 6.8 | 3.3 |  |  |  |  |
Specialty | 2018 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 10.3 | 9.8 | 8.1 | 2.9 |  |  |  |  |  |
Specialty | 2019 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 7.6 | 7.1 | 4.9 |  |  |  |  |  |  |
Specialty | 2020 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 10.9 | 5.4 |  |  |  |  |  |  |  |
Specialty | 2021 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 5.2 |  |  |  |  |  |  |  |  |
Casualty - Active |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 45.0 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2012, net of reinsurance | 0.2 |  |  |  |  |  |  |  |  |
Loss and LAE reserves, net of reinsurance | 37.4 |  |  |  |  |  |  |  |  |
Casualty - Active | 2012 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 9.1 | 8.9 | 9.1 | 8.5 | 8.0 | 7.4 | 6.4 | 4.8 | 3.3 | 1.3
Casualty - Active | 2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 7.2 | 7.0 | 7.0 | 6.7 | 6.3 | 5.7 | 5.3 | 3.6 | 1.5 |
Casualty - Active | 2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 6.0 | 5.9 | 5.5 | 5.2 | 4.7 | 4.2 | 3.5 | 1.3 |  |
Casualty - Active | 2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 5.1 | 4.9 | 4.7 | 4.4 | 3.2 | 2.4 | 1.8 |  |  |
Casualty - Active | 2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 5.3 | 4.6 | 4.0 | 2.3 | 1.0 | 0.2 |  |  |  |
Casualty - Active | 2017 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 4.2 | 3.4 | 2.8 | 1.7 | 0.8 |  |  |  |  |
Casualty - Active | 2018 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 4.3 | 3.5 | 1.4 | 0.3 |  |  |  |  |  |
Casualty - Active | 2019 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 2.3 | 1.4 | 0.3 |  |  |  |  |  |  |
Casualty - Active | 2020 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 1.0 | 0.5 |  |  |  |  |  |  |  |
Casualty - Active | 2021 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 0.5 |  |  |  |  |  |  |  |  |
Casualty - Runoff |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 216.9 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2012, net of reinsurance | 21.4 |  |  |  |  |  |  |  |  |
Loss and LAE reserves, net of reinsurance | 68.4 |  |  |  |  |  |  |  |  |
Casualty - Runoff | 2012 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 21.8 | 21.2 | 20.8 | 20.5 | 19.3 | 17.8 | 16.5 | 13.6 | 10.4 | $ 3.5
Casualty - Runoff | 2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 46.0 | 45.6 | 45.0 | 44.3 | 42.7 | 41.0 | 35.9 | 19.5 | $ 7.2 |
Casualty - Runoff | 2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 49.5 | 48.7 | 47.0 | 43.1 | 36.5 | 29.1 | 23.2 | $ 6.5 |  |
Casualty - Runoff | 2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 28.2 | 26.5 | 23.9 | 20.3 | 14.0 | 7.9 | $ 4.3 |  |  |
Casualty - Runoff | 2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 26.8 | 24.5 | 21.8 | 16.8 | 9.7 | $ 3.8 |  |  |  |
Casualty - Runoff | 2017 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 21.1 | 18.3 | 14.2 | 9.1 | $ 3.1 |  |  |  |  |
Casualty - Runoff | 2018 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 14.3 | 12.2 | 7.2 | $ 3.3 |  |  |  |  |  |
Casualty - Runoff | 2019 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 7.4 | 5.6 | $ 3.2 |  |  |  |  |  |  |
Casualty - Runoff | 2020 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 1.3 | $ 0.8 |  |  |  |  |  |  |  |
Casualty - Runoff | 2021 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | $ 0.5 |  |  |  |  |  |  |  |  |

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