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Schedule I - Investments (Details) - USD ($) $ in Millions | Dec. 31, 2024 | Dec. 31, 2023
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SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Fair Value | $ 6,589.3 |
Amount at which shown at the balance sheet | 6,741.5 |
Privately-held Investments | 286.8 | $ 475.0
Other Investments | 267.2 | 209.3
Fair Value, Recurring [Member] | Apollo originating partnership |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Privately-held Investments | 73.6 |
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 | 78.6 |
Fair Value, Recurring [Member] | Apollo originating partnership |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Privately-held Investments | 73.6 | $ 112.4
U.S. government |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Amortized Cost or Cost | 1,767.7 |
Fair Value | 1,741.9 |
Amount at which shown at the balance sheet | 1,741.9 |
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 | 85.8 |
Fair Value | 83.9 |
Amount at which shown at the balance sheet | 83.9 |
Corporate |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Amortized Cost or Cost | 2,200.8 |
Fair Value | 2,137.5 |
Amount at which shown at the balance sheet | 2,137.5 |
High yield loans |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Amortized Cost or Cost | 101.7 |
Fair Value | 102.4 |
Amount at which shown at the balance sheet | 102.4 |
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 | 135.7 |
Fair Value | 134.1 |
Amount at which shown at the balance sheet | 134.1 |
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 | 272.6 |
Fair Value | 271.2 |
Amount at which shown at the balance sheet | 271.2 |
Asset-backed |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Amortized Cost or Cost | 767.7 |
Fair Value | 770.8 |
Amount at which shown at the balance sheet | 859.7 |
Asset-backed | Fair Value, Recurring [Member] | Apollo originating partnership |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Other Investments | 88.9 |
Non-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 | 5.0 |
Fair Value | 4.4 |
Amount at which shown at the balance sheet | 4.4 |
Agency mortgage-backed |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Amortized Cost or Cost | 635.0 |
Fair Value | 549.8 |
Amount at which shown at the balance sheet | 549.8 |
Fixed maturities |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Amortized Cost or Cost | 5,979.5 |
Fair Value | 5,803.2 |
Amount at which shown at the balance sheet | 5,892.1 |
Short-term investments — Available for sale |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Amortized Cost or Cost | 262.9 |
Fair Value | 262.9 |
Amount at which shown at the balance sheet | 262.9 |
Catastrophe bonds, trading |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Amortized Cost or Cost | 1.0 |
Fair Value | 1.0 |
Amount at which shown at the balance sheet | 1.0 |
Privately-held investments |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Amortized Cost or Cost | 312.0 |
Fair Value | 237.4 |
Amount at which shown at the balance sheet | 311.0 |
Equity Method Investments |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Fair Value | 7.3 |
Amount at which shown at the balance sheet | 7.3 |
Other investments |  |
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items] |  |
Fair Value | 188.6 |
Amount at which shown at the balance sheet | $ 267.2 |

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