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Reserves for Losses and Adjustment Expenses - Additional Information (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Financial Guarantee Insurance Contracts, Premium Received over Contract Period [Line Items]
Prior years	$ 39.7	$ 10.5	$ 60.8	$ 28.1	$ 45.1
Reinsurance
Financial Guarantee Insurance Contracts, Premium Received over Contract Period [Line Items]
Prior years	21.5	(28.3)
Reinsurance | Reinsurance Contracts [Axis]: LPT, accident years 2020 onwards
Financial Guarantee Insurance Contracts, Premium Received over Contract Period [Line Items]
Prior years	(2.6)	1.2
Reinsurance | Reinsurance Contracts [Axis]: LPT, deferred gain
Financial Guarantee Insurance Contracts, Premium Received over Contract Period [Line Items]
Prior years	24.1	(29.5)
Insurance
Financial Guarantee Insurance Contracts, Premium Received over Contract Period [Line Items]
Prior years	18.2	38.8
Insurance | Reinsurance Contracts [Axis]: LPT, deferred gain
Financial Guarantee Insurance Contracts, Premium Received over Contract Period [Line Items]
Prior years	17.6	23.7
Insurance | Reinsurance Contracts [Axis]: LPT, offset post-LPT years
Financial Guarantee Insurance Contracts, Premium Received over Contract Period [Line Items]
Prior years	$ 0.6	$ 15.1

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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 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

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