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Fair Value of Marketable Securities (Detail) - USD ($) $ in Millions | Mar. 31, 2024 | Dec. 31, 2023 | Mar. 31, 2023
--- | --- | --- | ---
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Trading Securities | $ 5.9 | $ 6.0 | $ 5.7
Deferred Compensation Plan [Member] |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Trading Securities | 1.8 | 1.3 | 0.9
Fair Value, Inputs, Level 1 |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Trading Securities | 5.9 | 6.0 | 5.7
Fair Value, Inputs, Level 1 | Deferred Compensation Plan [Member] |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Trading Securities | 1.8 | 1.3 | 0.9
Fair Value, Inputs, Level 1 | Equity Funds | Deferred Compensation Plan [Member] |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Trading Securities | 1.6 | 1.1 | 0.8
Fair Value, Inputs, Level 1 | Fixed Income Funds [Member] |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Trading Securities | 3.9 | 4.0 | 0.0
Fair Value, Inputs, Level 1 | Fixed Income Funds [Member] | Deferred Compensation Plan [Member] |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Trading Securities | 0.1 | 0.1 | 0.0
Fair Value, Inputs, Level 1 | Money Market Funds |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Trading Securities | 2.0 | 2.0 | 5.7
Fair Value, Inputs, Level 1 | Money Market Funds | Deferred Compensation Plan [Member] |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |
Trading Securities | $ 0.1 | $ 0.1 | $ 0.1

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