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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 | Dec. 31, 2025 | Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
--- | --- | --- | --- | ---
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 | $ 1,972.7 | $ 1,693.1 | $ 1,264.3 | $ 791.5
Reinsurance recoverable on unpaid losses | (508.3) | (434.4) | (340.8) | (505.0)
Loss and loss adjustment expense reserves | 2,481.0 | 2,127.5 | $ 1,605.1 | $ 1,296.5
Ark Insurance Holdings Limited/Outrigger |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Reinsurance recoverable on unpaid losses | (508.3) | (434.4) |  |
Reinsurance recoverables on paid losses | 117.6 | 57.5 |  |
Ark Insurance Holdings Limited/Outrigger | Ark |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Reinsurance recoverable on unpaid losses | 29.5 | 31.8 |  |
Reinsurance recoverables on paid losses | (5.2) | (3.1) |  |
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 | 573.1 | 533.6 |  |
Total Reinsurance recoverables on unpaid losses | 142.4 | 121.5 |  |
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 | 569.0 | 463.1 |  |
Total Reinsurance recoverables on unpaid losses | 208.9 | 180.1 |  |
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 | 508.2 | 417.0 |  |
Total Reinsurance recoverables on unpaid losses | 44.4 | 51.3 |  |
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 | 230.6 | 178.0 |  |
Total Reinsurance recoverables on unpaid losses | 113.0 | 80.7 |  |
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 | 57.1 | 66.5 |  |
Total Reinsurance recoverables on unpaid losses | (0.4) | 0.8 |  |
WM Outrigger Re | 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 | $ 34.7 | $ 34.9 |  |

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