Components of Pension Expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]
Service cost	$ 94.0	$ 148.5	$ 148.4
Interest on projected benefit obligation	127.5	84.9	65.3
Expected return on assets	$ (230.3)	$ (215.1)	$ (203.3)
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) Excluding Service Cost, Statement of Income or Comprehensive Income [Extensible Enumeration]	Costs and Expenses	Costs and Expenses	Costs and Expenses
Amortization of prior service costs	$ 1.4	$ 0.7	$ 0.8
Recognized actuarial loss	4.7	28.9	58.7
Net pension (gain) expense	$ (2.7)	$ 47.9	$ 69.9