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Schedule I - Investments (Details) - USD ($) $ in Millions | Sep. 30, 2024 | Dec. 31, 2023 | Dec. 31, 2022
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SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |  |
Fair Value |  | $ 6,319.8 |
Amount at which shown at the balance sheet |  | 6,412.4 |
Privately-held Investments, trading | $ 341.2 | 475.0 | $ 533.0
Other investments, at fair value | 204.7 | 209.3 | 221.3
Fair Value, Recurring [Member] | Apollo Origination Partnership |  |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |  |
Privately-held Investments, trading |  | 112.4 |
Other investments, at fair value |  | 15.9 |
Fair Value, Recurring [Member] | Apollo Real Estate Fund |  |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |  |
Other investments, at fair value | $ 20.1 | 23.9 | $ 25.3
U.S. government |  |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |  |
Amortized Cost or Cost |  | 1,473.6 |
Fair Value |  | 1,448.1 |
Amount at which shown at the balance sheet |  | 1,448.1 |
U.S. agency |  |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |  |
Amortized Cost or Cost |  | 7.5 |
Fair Value |  | 7.2 |
Amount at which shown at the balance sheet |  | 7.2 |
Municipal |  |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |  |
Amortized Cost or Cost |  | 136.9 |
Fair Value |  | 131.2 |
Amount at which shown at the balance sheet |  | 131.2 |
Corporate |  |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |  |
Amortized Cost or Cost |  | 2,229.9 |
Fair Value |  | 2,130.8 |
Amount at which shown at the balance sheet |  | 2,130.8 |
High yield loans |  |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |  |
Amortized Cost or Cost |  | 90.8 |
Fair Value |  | 92.1 |
Amount at which shown at the balance sheet |  | 92.1 |
Non-U.S. government-backed corporate |  |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |  |
Amortized Cost or Cost |  | 115.1 |
Fair Value |  | 109.0 |
Amount at which shown at the balance sheet |  | 109.0 |
Non-U.S government |  |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |  |
Amortized Cost or Cost |  | 315.7 |
Fair Value |  | 308.6 |
Amount at which shown at the balance sheet |  | 308.6 |
Asset-backed |  |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |  |
Amortized Cost or Cost |  | 936.0 |
Fair Value |  | 908.2 |
Amount at which shown at the balance sheet |  | 908.2 |
Agency commercial mortgage-backed securities |  |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |  |
Amortized Cost or Cost |  | 6.6 |
Fair Value |  | 5.8 |
Amount at which shown at the balance sheet |  | 5.8 |
Agency mortgage-backed |  |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |  |
Amortized Cost or Cost |  | 544.9 |
Fair Value |  | 467.3 |
Amount at which shown at the balance sheet |  | 467.3 |
Fixed maturities |  |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |  |
Amortized Cost or Cost |  | 5,857.0 |
Fair Value |  | 5,608.3 |
Amount at which shown at the balance sheet |  | 5,608.3 |
Short-term investments |  |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |  |
Amortized Cost or Cost |  | 95.7 |
Fair Value |  | 95.7 |
Amount at which shown at the balance sheet |  | 95.7 |
Catastrophe Bonds |  |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |  |
Amortized Cost or Cost |  | 1.6 |
Fair Value |  | 1.6 |
Amount at which shown at the balance sheet |  | 1.6 |
Privately-held investments |  |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |  |
Amortized Cost or Cost |  | 509.6 |
Fair Value |  | 437.1 |
Amount at which shown at the balance sheet |  | 489.9 |
Equity Method Investments |  |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |  |
Fair Value |  | 7.6 |
Amount at which shown at the balance sheet |  | 7.6 |
Other investments |  |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |  |
Fair Value |  | 169.6 |
Amount at which shown at the balance sheet |  | $ 209.3 |

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