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Investments - Net Investment Losses (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
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Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Gain (Loss) on Securities [Line Items] |  |  |  |
Net investment gains (losses) | $ 3.3 | $ 3.8 | $ (5.9) | $ 0.1
Fixed maturity securities |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |
Net investment gains (losses) | (0.6) | (0.3) | (4.5) | (3.6)
Equity securities |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |
Net investment gains (losses) | 2.2 | 5.1 | 0.2 | 5.6
Short-term investments and other |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |
Net investment gains (losses) | $ 1.7 | $ (1.0) | $ (1.6) | $ (1.9)

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