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Loss and Loss Adjustment Expense Reserves - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended |  |  |  |
--- | --- | --- | --- | --- | ---
Mar. 31, 2024 | Mar. 31, 2023 | Dec. 31, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Net incurred loss and loss adjustment expenses | $ 185.8 | $ 147.8 |  |  |  |
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%
P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Prior year losses | (0.3) | 8.6 |  |  |  |
Loss and loss adjustment expense reserves | 1,744.5 | 1,345.6 | $ 1,605.1 | $ 1,296.5 |  |
Net incurred loss and loss adjustment expenses | 180.0 | 147.8 |  |  |  |
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 | 1,744.5 |  | 1,605.1 |  |  |
Net incurred loss and loss adjustment expenses | 180.0 | 147.8 |  |  |  |
P&C Insurance Distribution (Bamboo) | Operating Segments |  |  |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |  |  |
Loss and loss adjustment expense reserves | 12.3 |  | $ 0.0 |  |  |
Net incurred loss and loss adjustment expenses | $ 5.8 | $ 0.0 |  |  |  |

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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 Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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- Definition Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred and costs incurred in the claim settlement process. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-3 + Details
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- Definition Amount, after effects of reinsurance, of expense (reversal of expense) for claims incurred in prior reporting periods and related claims settlement costs. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 944 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480081/944-40-50-3 + Details
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- Definition Reinsurance, Loss and Loss Reserves, Participation Results, Percentage + References No definition available. + Details
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