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Loss and Loss Adjustment Expense Reserves - Cumulative Paid Loss and Allocated LAE, Net of Reinsurance (Details) - Ark - USD ($) $ in Millions | Dec. 31, 2022 | 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
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Property and Accident & Health |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | $ 410.2 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2013, net of reinsurance | 1.4 |  |  |  |  |  |  |  |  |
Loss and LAE reserves, net of reinsurance | 258.2 | $ 175.0 |  |  |  |  |  |  |  |
Property and Accident & Health | 2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 59.2 | 59.2 | $ 59.3 | $ 59.3 | $ 59.4 | $ 59.1 | $ 59.1 | $ 58.1 | $ 39.1 | $ 15.4
Property and Accident & Health | 2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 27.9 | 27.8 | 27.9 | 27.8 | 27.6 | 27.5 | 27.1 | 24.9 | 13.6 |
Property and Accident & Health | 2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 15.0 | 15.0 | 14.8 | 14.6 | 14.6 | 13.4 | 12.2 | 6.9 |  |
Property and Accident & Health | 2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 17.8 | 17.2 | 16.9 | 16.8 | 16.4 | 13.1 | 8.5 |  |  |
Property and Accident & Health | 2017 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 27.3 | 29.6 | 32.8 | 31.6 | 25.8 | 16.8 |  |  |  |
Property and Accident & Health | 2018 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 40.8 | 40.0 | 40.1 | 32.2 | 15.6 |  |  |  |  |
Property and Accident & Health | 2019 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 18.5 | 18.3 | 16.7 | 6.8 |  |  |  |  |  |
Property and Accident & Health | 2020 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 47.0 | 34.1 | 11.2 |  |  |  |  |  |  |
Property and Accident & Health | 2021 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 86.7 | 30.8 |  |  |  |  |  |  |  |
Property and Accident & Health | 2022 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 70.0 |  |  |  |  |  |  |  |  |
Marine & Energy |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 164.0 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2013, net of reinsurance | 4.0 |  |  |  |  |  |  |  |  |
Loss and LAE reserves, net of reinsurance | 196.4 | 99.3 |  |  |  |  |  |  |  |
Marine & Energy | 2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 29.3 | 29.3 | 29.1 | 29.3 | 29.3 | 29.1 | 28.6 | 27.6 | 22.2 | 7.8
Marine & Energy | 2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 13.7 | 13.5 | 13.6 | 13.4 | 14.1 | 14.0 | 13.2 | 12.1 | 5.8 |
Marine & Energy | 2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 11.4 | 10.8 | 10.4 | 10.3 | 10.9 | 9.6 | 7.8 | 4.0 |  |
Marine & Energy | 2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 13.4 | 13.7 | 13.1 | 13.0 | 12.6 | 10.0 | 5.5 |  |  |
Marine & Energy | 2017 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 14.1 | 14.1 | 14.0 | 12.8 | 11.1 | 5.1 |  |  |  |
Marine & Energy | 2018 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 15.4 | 14.7 | 14.0 | 12.5 | 2.7 |  |  |  |  |
Marine & Energy | 2019 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 14.3 | 12.6 | 10.6 | 3.3 |  |  |  |  |  |
Marine & Energy | 2020 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 16.0 | 12.7 | 3.1 |  |  |  |  |  |  |
Marine & Energy | 2021 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 24.2 | 6.3 |  |  |  |  |  |  |  |
Marine & Energy | 2022 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 12.2 |  |  |  |  |  |  |  |  |
Specialty |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 165.8 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2013, net of reinsurance | 0.2 |  |  |  |  |  |  |  |  |
Loss and LAE reserves, net of reinsurance | 204.3 | 85.2 |  |  |  |  |  |  |  |
Specialty | 2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 15.6 | 15.6 | 15.7 | 15.7 | 15.7 | 15.5 | 15.4 | 14.9 | 13.2 | 17.0
Specialty | 2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 43.0 | 42.7 | 42.8 | 42.0 | 40.7 | 40.1 | 39.7 | 38.9 | 26.3 |
Specialty | 2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 6.4 | 8.1 | 8.1 | 8.1 | 8.0 | 7.6 | 7.0 | 4.0 |  |
Specialty | 2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 8.5 | 10.3 | 10.3 | 9.9 | 9.1 | 7.9 | 3.2 |  |  |
Specialty | 2017 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 9.2 | 8.5 | 8.5 | 8.4 | 6.6 | 3.1 |  |  |  |
Specialty | 2018 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 11.8 | 10.4 | 10.0 | 8.2 | 2.7 |  |  |  |  |
Specialty | 2019 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 18.2 | 7.4 | 6.9 | 4.8 |  |  |  |  |  |
Specialty | 2020 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 13.0 | 10.6 | 5.2 |  |  |  |  |  |  |
Specialty | 2021 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 24.1 | 5.1 |  |  |  |  |  |  |  |
Specialty | 2022 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 16.0 |  |  |  |  |  |  |  |  |
Casualty - Active |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 42.0 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2013, net of reinsurance | 4.0 |  |  |  |  |  |  |  |  |
Loss and LAE reserves, net of reinsurance | 71.5 | 37.4 |  |  |  |  |  |  |  |
Casualty - Active | 2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 7.5 | 7.3 | 7.0 | 7.0 | 6.7 | 6.3 | 5.8 | 5.3 | 3.6 | 1.5
Casualty - Active | 2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 6.2 | 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 | 5.5 | 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 | 6.5 | 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 | 5.7 | 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 | 4.3 | 3.5 | 1.4 | 0.3 |  |  |  |  |
Casualty - Active | 2019 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 3.0 | 2.3 | 1.4 | 0.3 |  |  |  |  |  |
Casualty - Active | 2020 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 2.0 | 1.0 | 0.5 |  |  |  |  |  |  |
Casualty - Active | 2021 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 0.9 | 0.5 |  |  |  |  |  |  |  |
Casualty - Active | 2022 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 0.4 |  |  |  |  |  |  |  |  |
Casualty - Runoff |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 214.8 |  |  |  |  |  |  |  |  |
All outstanding liabilities before 2013, net of reinsurance | 19.4 |  |  |  |  |  |  |  |  |
Loss and LAE reserves, net of reinsurance | 60.8 | 68.4 |  |  |  |  |  |  |  |
Casualty - Runoff | 2013 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 45.2 | 44.9 | 44.6 | 43.9 | 43.3 | 42.4 | 40.6 | 35.7 | 19.4 | $ 7.1
Casualty - Runoff | 2014 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 51.8 | 49.3 | 48.5 | 46.9 | 43.1 | 36.4 | 29.5 | 23.1 | $ 6.4 |
Casualty - Runoff | 2015 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 33.1 | 28.9 | 27.3 | 24.7 | 21.4 | 14.5 | 8.2 | $ 4.3 |  |
Casualty - Runoff | 2016 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 28.7 | 27.8 | 25.4 | 22.7 | 17.7 | 10.2 | $ 3.9 |  |  |
Casualty - Runoff | 2017 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 22.5 | 21.4 | 18.5 | 14.6 | 9.4 | $ 3.2 |  |  |  |
Casualty - Runoff | 2018 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 16.3 | 14.9 | 12.6 | 7.4 | $ 3.4 |  |  |  |  |
Casualty - Runoff | 2019 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 12.1 | 7.8 | 5.8 | $ 3.3 |  |  |  |  |  |
Casualty - Runoff | 2020 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 3.1 | 1.3 | $ 0.8 |  |  |  |  |  |  |
Casualty - Runoff | 2021 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | 1.7 | $ 0.5 |  |  |  |  |  |  |  |
Casualty - Runoff | 2022 |  |  |  |  |  |  |  |  |  |
Claims Development [Line Items] |  |  |  |  |  |  |  |  |  |
Cumulative Paid Loss and LAE, Net of Reinsurance | $ 0.3 |  |  |  |  |  |  |  |  |

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