Stockholder's Equity - Accumulated Other Comprehensive Income (Loss) - Reclassifications of Accumulated Other Comprehensive Loss (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
Amortization of pension and postretirement items:
Tax benefit	$ (18.4)	$ (16.6)
Derivative, Gain (Loss) on Derivative, Net	0.8	1.3
Proceeds from Issuance of Common Stock, Dividend Reinvestment Plan	19.8	0.1
Interest Rate Swap [Member] | Derivatives designated as hedging instrument [Member] | Interest Expense [Member]
Amortization of pension and postretirement items:
Derivative, Gain (Loss) on Derivative, Net	$ 0.1	$ 0.2