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Investment Securities - Investment Gains (Losses) for Level 3 (Details) - Other long-term investments - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2024 | Sep. 30, 2023 | Sep. 30, 2024 | Sep. 30, 2023
Gain (Loss) on Securities [Line Items] |  |  |  |
Net unrealized investment gains (losses) | $ 49.2 | $ 34.2 | $ 89.9 | $ 108.6
Level 3 |  |  |  |
Gain (Loss) on Securities [Line Items] |  |  |  |
Net unrealized investment gains (losses) | $ 31.3 | $ 24.6 | $ 82.6 | $ 76.0

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