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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] |  |  |  |  |  |  |
Investment income |  |  |  |  | $ (287.3) | $ (198.4) | $ (159.2)
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] |  |  |  |  |  |  |
Investment income |  |  |  |  | (115.7) | (95.6) | (87.2)
Short-term investments |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |
Investment income |  |  |  |  | (5.3) | (0.6) | (0.1)
Short-term investment, Trading |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |
Investment income |  |  |  |  | (0.3) | (0.1) | 0.0
Fixed term deposits (included in cash and cash equivalents) |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |
Investment income |  |  |  |  | (39.9) | (6.6) | (0.7)
Catastrophe bonds — Trading |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |
Investment income |  |  |  |  | (0.2) | (0.3) | (0.9)
Privately-held investments, available for sale |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |
Investment income |  |  |  |  | (0.1) | 0.0 | 0.0
Privately-held investments — Trading |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |
Investment income |  |  |  |  | (44.7) | (24.3) | (18.2)
Other investments, at fair value |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |
Investment income |  |  |  |  | (17.8) | (13.9) | (21.9)
Fixed Income Securities |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |
Investment income |  |  |  |  | $ (98.9) | $ (57.0) | $ (30.2)

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- Definition Represents the total of interest and dividend income, including any amortization and accretion (as applicable) of discounts and premiums, earned from (1) loans and leases whether held-for-sale or held-in-portfolio; (2) investment securities; (3) federal funds sold; (4) securities purchased under agreements to resell; (5) investments in banker's acceptances, commercial paper, or certificates of deposit; (6) dividend income; or (7) other investments not otherwise specified herein. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 946 -SubTopic 830 -Name Accounting Standards Codification -Section 45 -Paragraph 39 -Publisher FASB -URI https://asc.fasb.org//1943274/2147480228/946-830-45-39 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 942 -SubTopic 220 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.1-5) -Publisher FASB -URI https://asc.fasb.org//1943274/2147483589/942-220-S99-1 + Details
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