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Investments - Net Investment Losses (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2025 | Mar. 31, 2024
Gain (Loss) on Securities |  |
Net investment gains (losses) | $ (3.3) | $ 2.2
Fixed maturity securities |  |
Gain (Loss) on Securities |  |
Net investment gains (losses) | 0.2 | (1.0)
Equity securities |  |
Gain (Loss) on Securities |  |
Net investment gains (losses) | (1.2) | 2.6
Short-term investments and other |  |
Gain (Loss) on Securities |  |
Net investment gains (losses) | $ (2.3) | $ 0.6

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