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Loss and Loss Adjustment Expense Reserves - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2025 | Mar. 31, 2024 | Dec. 31, 2024 | Dec. 31, 2023
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Loss and loss adjustment expenses | $ 261.8 | $ 185.8 |  |
Other Operations |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Loss and loss adjustment expense reserves | 13.4 |  | $ 12.1 |
Loss and loss adjustment expenses | 17.4 | 0.0 |  |
P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Prior year losses | (52.8) | (0.3) |  |
Loss and loss adjustment expense reserves | 2,253.9 | 1,744.5 | 2,127.5 | $ 1,605.1
Loss and loss adjustment expenses | 233.5 | 180.0 |  |
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 | 2,253.9 |  | 2,127.5 |
Loss and loss adjustment expenses | 233.5 | 180.0 |  |
P&C Insurance Distribution (Bamboo) |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Loss and loss adjustment expenses | 10.9 | 5.8 |  |
P&C Insurance Distribution (Bamboo) | Operating Segments |  |  |  |
Liability for Claims and Claims Adjustment Expense [Line Items] |  |  |  |
Loss and loss adjustment expense reserves | 23.2 |  | $ 17.8 |
Loss and loss adjustment expenses | $ 10.9 | $ 5.8 |  |

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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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- 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://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/2009/role/commonPracticeRef -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/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/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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