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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 | [1] | 286.8 | $ 475.0
Other Investments | [2] | 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 |
[1] | Privately-held investments, trading at fair value include related party investments totaling $73.6 million (2023 — $112.4 million). Privately-held investments, available for sale include related party investments totaling $Nil (2023 — $14.9 million).
[2] | Other investments includes related party investments in Funds Managed by Apollo of $78.6 million (2023 — $39.8 million).

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