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Investment Securities - Debt Securities Issued By Corporation (Details) - Debt securities issued by corporations - USD ($) $ in Millions | Dec. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Investment securities |  |
Fixed maturity investments, at fair value | $ 1,414.2 | $ 1,045.0
AAA |  |
Investment securities |  |
Fixed maturity investments, at fair value | 14.6 | 11.5
AA |  |
Investment securities |  |
Fixed maturity investments, at fair value | 91.2 | 83.8
A |  |
Investment securities |  |
Fixed maturity investments, at fair value | 607.2 | 552.4
BBB |  |
Investment securities |  |
Fixed maturity investments, at fair value | 688.7 | 390.9
BB |  |
Investment securities |  |
Fixed maturity investments, at fair value | 5.4 | 0.0
Other |  |
Investment securities |  |
Fixed maturity investments, at fair value | $ 7.1 | $ 6.4

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