Equity Securities - (Summary of Investments in Marketable Equity Securities) (Detail) - USD ($) $ in Millions |
Sep. 30, 2026 |
Dec. 31, 2025 |
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| Schedule Of Trading Securities And Other Trading Assets [Line Items] | ||
| Equity securities, Cost | $ 466.1 | $ 466.9 |
| Equity securities, Unrealized gains | 39.3 | 10.7 |
| Equity securities, Estimated fair value | 505.4 | 477.6 |
| Common Stock | ||
| Schedule Of Trading Securities And Other Trading Assets [Line Items] | ||
| Equity securities, Cost | 457.5 | 457.9 |
| Equity securities, Unrealized gains | 38.1 | 9.8 |
| Equity securities, Estimated fair value | 495.6 | 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 |
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- Definition Amount of unrealized gain (loss) on investment in marketable equity securities. No definition available.
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- Definition Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI), classified as current. Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value. Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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