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Loss and Loss Adjustment Expense Reserves - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Loss adjustment expense reserves, loss on delinquent payment | $ 0.3 |  |  |  |  |
Loss and loss adjustment expenses | 888.5 | $ 726.8 | $ 536.4 |  |  |
Other Operations |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Loss and loss adjustment expense reserves | 12.1 | 0.0 |  |  |  |
Loss and loss adjustment expenses | 12.1 | 0.0 | 0.0 |  |  |
P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Favorable prior loss reserve development | (58.6) | 19.9 | (51.7) |  |  |
Loss and loss adjustment expense reserves | 2,127.5 | 1,605.1 | 1,296.5 | $ 894.7 |  |
Loss and loss adjustment expenses | 855.8 | 726.8 | 536.4 |  |  |
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Loss and loss adjustment expense reserves | 2,127.5 | 1,605.1 |  |  |  |
Loss and loss adjustment expenses | 855.8 | 726.8 | 536.4 |  |  |
P&C Insurance Distribution (Bamboo) |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Loss and loss adjustment expenses | 20.6 |  |  |  |  |
P&C Insurance Distribution (Bamboo) | Operating Segments |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Loss and loss adjustment expense reserves | 17.8 | 0.0 |  |  |  |
Loss and loss adjustment expenses | 20.6 | 0.0 | 0.0 |  |  |
Syndicate - Ark Insurance Holdings Limited |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Percent of underwriting capital provided by third parties |  |  |  | 0.00% | 42.80% | 58.30%
Property and Accident & Health | P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Favorable prior loss reserve development | (29.2) | 41.7 | (20.8) |  |  |
Specialty | P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Favorable prior loss reserve development | (34.3) | 11.9 | (12.7) |  |  |
Casualty-Runoff | P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Favorable prior loss reserve development |  | $ 6.0 |  |  |  |
Marine & Energy | P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Favorable prior loss reserve development |  |  | $ (18.8) |  |  |
Casualty-Active | P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Favorable prior loss reserve development | $ 4.5 |  |  |  |  |

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- Definition The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 7 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480046/944-40-55-7 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 55 -Paragraph 9E -Publisher FASB -URI https://asc.fasb.org/1943274/2147480046/944-40-55-9E Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (cc) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480081/944-40-50-3 Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 944 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(13)(a)(1)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1 + Details
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