Components of Pension Expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Net Periodic Benefit Cost [Abstract]
Service cost	$ 130.4	$ 102.0	$ 108.3
Interest on projected benefit obligation	83.0	95.6	85.8
Expected return on assets	(190.6)	(176.6)	(177.2)
Amortization of prior service costs	1.5	1.4	1.4
Recognized actuarial loss	44.2	20.5	35.3
Settlement loss	11.6
Net pension expense	$ 80.1	$ 42.9	$ 53.6