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Investments - Net Realized and Unrealized Investment Gains and Losses and Change in Unrealized Gains and Losses on Investments (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] |  |  |  |  |  |  |
Fixed income securities - gross realized gains | $ 1.0 | $ 0.0 | $ 1.4 | $ 0.4 | $ 1.6 | $ 2.9 | $ 22.7
Fixed income securities - gross realizes (losses) | (10.9) | (8.6) | (36.1) | (15.6) | (41.5) | (58.4) | (3.6)
Net change in expected credit gains (losses) | 0.5 | 0.3 | 0.9 | 4.0 | 4.8 | (5.0) | (2.5)
Fixed income securities, trading - gross realized gains |  |  |  |  | 1.0 | 0.2 | 12.2
Fixed income securities, trading, gross realizes (losses) |  |  |  |  | (3.5) | (1.8) | (2.0)
Privately-held investments net unrealized gains/(losses) |  |  |  |  | 65.9 | (113.9) | (23.4)
Catastrophe bonds | 0.0 | 0.1 | 0.0 | (0.5) | 0.1 | 0.2 | (0.8)
Total net realized and unrealized investment (losses)/gains recorded in the consolidated statement of operations | 6.1 | (1.8) | (21.0) | 16.2 | 14.5 | (177.6) | 8.8
Change in available for sale net unrealized (losses)/gains: |  |  |  |  |  |  |
Fixed income securities | 120.7 | (45.0) | 103.6 | (32.7) | 126.2 | (391.7) | (157.6)
Income tax benefit/(expense) |  |  |  |  | (20.6) | 23.9 | (0.3)
Total change in net unrealized (losses)/gains, net of taxes recorded in other comprehensive income |  |  |  |  | 105.6 | (367.8) | (157.9)
Privately-held investments |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |
Fixed income securities, trading - gross realized gains | 0.1 | 0.2 | 0.6 | 0.3 | 0.8 | 0.7 | 0.6
Fixed income securities, trading, gross realizes (losses) | (14.3) | 0.0 | (14.6) | 0.0 | 0.0 | (0.1) | (13.8)
Privately-held investments net unrealized gains/(losses) | 9.5 | (4.1) | (8.0) | (13.1) | (15.2) | (2.5) | 18.1
Short-term investments |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |
Fixed income securities - gross realized gains | 0.5 | 0.0 | 0.7 | 0.1 | 0.6 | 1.0 | 2.0
Fixed income securities - gross realizes (losses) | 0.0 | 0.0 | (1.4) | (0.3) | (0.9) | (0.5) | (0.8)
Fixed income securities, trading - gross realized gains | 0.0 | 0.0 | 0.0 | 0.1 | 0.1 | 0.0 | 0.1
Fixed income securities, trading, gross realizes (losses) | 0.0 | 0.0 | (0.2) | (0.2) | (0.3) | 0.0 | (0.3)
MVI |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |
Income/(loss) from equity method investment |  |  |  |  | 0.2 | 0.0 | 0.1
Multi-Line Insurer |  |  |  |  |  |  |
Schedule of Investment Income, Reported Amounts, by Category [Line Items] |  |  |  |  |  |  |
Income/(loss) from equity method investment | $ 0.3 | $ 0.4 | $ 0.5 | $ 0.8 | $ 0.8 | $ (0.4) | $ 0.2

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