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Loss and Loss Adjustment Expense Reserves - Schedule of Ending Liabilities for Unpaid Loss and LAE, Net of Reinsurance (Details) - USD ($) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021
P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses | $ 1,693.1 | $ 1,264.3 | $ 791.5 | $ 465.8
Reinsurance recoverable on unpaid losses | (434.4) | (340.8) | (505.0) | (428.9)
Loss and loss adjustment expense reserves | 2,127.5 | 1,605.1 | 1,296.5 | 894.7
Ark Insurance Holdings Limited/Outrigger |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Reinsurance recoverable on unpaid losses | (434.4) | (340.8) |  |
Ceded | 200.9 | 55.7 | 278.5 |
Ark Insurance Holdings Limited/Outrigger | Ark Insurance Holdings Limited - Segment |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Reinsurance recoverable on unpaid losses | 31.8 | 15.6 |  |
Ceded | (29.9) | (15.6) |  |
Amount attributable to TPC Providers | P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Reinsurance recoverable on unpaid losses |  |  | $ (145.4) | $ (276.8)
Property and Accident & Health | P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses | 533.6 | 358.7 |  |
Total Reinsurance recoverables on unpaid losses | 121.5 | 126.9 |  |
Marine & Energy | P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses | 463.1 | 331.7 |  |
Total Reinsurance recoverables on unpaid losses | 180.1 | 59.1 |  |
Specialty | P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses | 417.0 | 339.7 |  |
Total Reinsurance recoverables on unpaid losses | 51.3 | 77.6 |  |
Casualty-Active | P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses | 178.0 | 137.1 |  |
Total Reinsurance recoverables on unpaid losses | 80.7 | 74.8 |  |
Casualty-Runoff | P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses | 66.5 | 81.3 |  |
Total Reinsurance recoverables on unpaid losses | 0.8 | 2.4 |  |
Other | P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses | $ 34.9 | $ 15.8 |  |

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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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