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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.

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Investments - Summary of Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Interest Income (Expense), Net	$ 81.7	$ 79.0	$ 248.1	$ 213.7
Investment expenses	(2.1)	(1.1)	(9.2)	(6.4)	$ (11.6)	$ (10.3)	$ (11.7)
Net investment income	79.6	77.9	238.9	207.3	$ 275.7	$ 188.1	$ 147.5
Fixed income securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Interest Income (Expense), Net	40.7	28.8	117.0	84.5
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Interest Income (Expense), Net	0.7	2.2	2.5	3.8
Short-term investment, Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Interest Income (Expense), Net	0.0	0.2	0.1	0.3
Fixed term deposits (included in cash and cash equivalents)
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Interest Income (Expense), Net	12.1	9.8	33.2	27.1
Catastrophe bonds — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Interest Income (Expense), Net	0.0	0.3	0.0	1.4
Privately-held investments, available for sale
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Interest Income (Expense), Net	0.3	0.0	0.7	0.0
Privately-held investments — Trading
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Interest Income (Expense), Net	8.7	11.4	29.7	32.9
Other investments, at fair value
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Interest Income (Expense), Net	2.3	(0.7)	4.4	9.7
Fixed Income Securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Interest Income (Expense), Net	$ 21.5	$ 25.6	$ 69.3	$ 73.4