Equity Securities - (Summary of Investments in Marketable Equity Securities) (Detail) - USD ($) $ in Millions |
Jun. 30, 2026 |
Dec. 31, 2025 |
|---|---|---|
| Schedule Of Trading Securities And Other Trading Assets [Line Items] | ||
| Equity securities, Cost | $ 456.3 | $ 466.9 |
| Equity securities, Unrealized gains | 38.2 | 10.7 |
| Equity securities, Estimated fair value | 494.5 | 477.6 |
| Common Stock | ||
| Schedule Of Trading Securities And Other Trading Assets [Line Items] | ||
| Equity securities, Cost | 447.7 | 457.9 |
| Equity securities, Unrealized gains | 37.0 | 9.8 |
| Equity securities, Estimated fair value | 484.7 | 467.7 |
| Preferred Stock | ||
| Schedule Of Trading Securities And Other Trading Assets [Line Items] | ||
| Equity securities, Cost | 8.6 | 9.0 |
| Equity securities, Unrealized gains | 1.2 | 0.9 |
| Equity securities, Estimated fair value | $ 9.8 | $ 9.9 |