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