Components of Net Periodic Benefit Expense Related to Post-Retirement Plans (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 4.7	$ 5.0
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	4.8	4.9
Interest cost	4.4	4.7
Expected return on plan assets	(3.8)	(4.1)
Amortization of net actuarial loss from earlier periods	1.6	2.4
Amortization of net prior service costs from earlier periods	(0.1)
Net periodic expense	6.9	7.9
Other Retirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.7	0.6
Interest cost	0.3	0.3
Amortization of net prior service costs from earlier periods	(0.1)	(0.1)
Net periodic expense	$ 0.9	$ 0.8