Components of Net Periodic Benefit Expense Related to Retirement Plans (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 18.4	$ 20.1	$ 21.6
Interest cost	18.5	18.2	16.9
Expected return on plan assets	(16.5)	(17.0)	(14.4)
Amortization of net actuarial loss from earlier periods	8.8	9.8	12.5
Amortization of net prior service costs from earlier periods		0.1	0.7
Net periodic expense	29.2	31.2	37.3
Other Retirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2.5	2.2	2.2
Interest cost	1.1	1.0	1.0
Amortization of net actuarial loss from earlier periods	0.1	0.2	0.3
Amortization of net prior service costs from earlier periods	(0.3)	(0.3)
Net periodic expense	$ 3.4	$ 3.1	$ 3.5