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Investments - Amortized Costs and Estimated Fair Value (Details) - USD ($) $ in Millions | Jun. 30, 2025 | Dec. 31, 2024
--- | --- | ---
Amortized Cost |  |
Due in One Year or Less | $ 259.7 |
Due after One Year to Five Years | 893.5 |
Due after Five Years to Ten Years | 981.3 |
Due after Ten Years | 3,524.7 |
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date | 1,825.1 |
Fixed maturities, at amortized cost | 7,484.3 | $ 7,295.0
Fair Value |  |
Due in One Year or Less | 255.7 |
Due after One Year to Five Years | 868.8 |
Due after Five Years to Ten Years | 885.7 |
Due after Ten Years | 2,946.2 |
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date | 1,712.7 |
Fair Value | $ 6,669.1 | $ 6,409.6

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