Components of Net Pension Expense (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Net Periodic Benefit Cost [Abstract]
Service cost	$ 25.3	$ 27.0	$ 77.1	$ 81.7
Interest on projected benefit obligation	23.7	21.3	71.7	64.3
Expected return on assets	(43.8)	(43.9)	(132.3)	(133.2)
Amortization of prior service costs	0.4	0.3	1.1	1.0
Recognized actuarial loss	5.0	8.7	15.3	26.5
Net pension expense	$ 10.6	$ 13.4	$ 32.9	$ 40.3