Investment Securities - Investment Gains (Losses) for Level 3 (Details) - Other long-term investments - USD ($) $ in Millions |
3 Months Ended | 9 Months Ended | ||
|---|---|---|---|---|
Sep. 30, 2026 |
Sep. 30, 2025 |
Sep. 30, 2026 |
Sep. 30, 2025 |
|
| Gain (Loss) on Securities [Line Items] | ||||
| Total net unrealized investment gains (losses) recognized during the period on Level 3 investments held at the end of period | $ (9.0) | $ 39.6 | $ 124.7 | $ 126.6 |
| Level 3 | ||||
| Gain (Loss) on Securities [Line Items] | ||||
| Total net unrealized investment gains (losses) recognized during the period on Level 3 investments held at the end of period | $ (2.8) | $ 42.0 | $ 121.0 | $ 103.6 |