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Loss and Loss Adjustment Expense Reserves - Schedule of Ending Liabilities for Unpaid Loss and LAE, Net of Reinsurance (Details) - Ark - USD ($) $ in Millions | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses | $ 1,264.3 | $ 791.5 | $ 465.8 | $ 262.6
Reinsurance recoverable on unpaid losses | (340.8) | (505.0) | (428.9) | (433.4)
Loss and loss adjustment expense reserves | 1,605.1 | 1,296.5 | 894.7 | 696.0
Ark |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Ceded | (15.6) |  |  |
Amount attributable to TPC Providers |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Reinsurance recoverable on unpaid losses | (0.0) | (145.4) | $ (276.8) | $ (319.2)
Property and Accident & Health |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses | 358.7 | 258.2 |  |
Total Reinsurance recoverables on unpaid losses | 126.9 | 224.6 |  |
Specialty |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses | 339.7 | 204.3 |  |
Total Reinsurance recoverables on unpaid losses | 77.6 | 97.2 |  |
Marine & Energy |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses | 331.7 | 196.4 |  |
Total Reinsurance recoverables on unpaid losses | 59.1 | 79.8 |  |
Casualty-Active |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses | 137.1 | 71.5 |  |
Total Reinsurance recoverables on unpaid losses | 74.8 | 49.9 |  |
Casualty-Runoff |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses | 81.3 | 60.8 |  |
Total Reinsurance recoverables on unpaid losses | 2.4 | 53.5 |  |
Other |  |  |  |
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items] |  |  |  |
Unpaid loss and LAE reserves, net of reinsurance recoverables on unpaid losses | $ 15.8 | $ 0.3 |  |

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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://fasb.org/us-gaap/role/ref/legacyRef -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 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-3 + Details
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- Definition Liability as of the balance sheet date for amounts representing estimated cost of settling unpaid claims under the terms of the underlying insurance policies, less estimated reinsurance recoveries on such claims. This includes an estimate for claims which have been incurred but not reported. Claim adjustment expenses represent the costs estimated to be incurred in the settlement of unpaid claims. + References Reference 1: 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//1943274/2147479440/944-210-S99-1 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -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 3: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-3 + Details
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