Components of Pension Expense (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Net Periodic Benefit Cost [Abstract]
Service cost	$ 92.9	$ 88.6	$ 91.3
Interest on projected benefit obligation	81.1	94.3	92.2
Expected return on assets	(159.7)	(141.7)	(140.8)
Amortization of prior service costs	1.2	1.2	1.3
Recognized actuarial loss	25.4	27.7	41.1
Curtailment gain			(0.1)
Net pension expense	$ 40.9	$ 70.1	$ 85.0