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Schedule I - Investments (Details) - USD ($)
$ in Millions
Sep. 30, 2024
Dec. 31, 2023
Dec. 31, 2022
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