Derivatives and Financial Instruments - Recorded Amounts and Estimated Fair Values (Detail) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Recorded Amount
Derivative [Line Items]
Variable rate debt	$ 825.0	$ 1,187.5
Fixed rate debt	819.3	819.3
Total debt	1,644.3	2,006.8
Fair Value
Derivative [Line Items]
Variable rate debt	825.0	1,187.5
Fixed rate debt	834.9	824.5
Total debt	$ 1,659.9	$ 2,012.0