Employee Benefits - Components of Estimated Net Pension and Other Postretirement Benefit Plan Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Pension expense (income)
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0.2	$ 0.2
Interest cost	7.8	7.8
Expected return on plan assets	(12.6)	(12.3)
Amortization, net	2.0	1.7
Settlements	(0.3)
Net pension income	(2.3)	(2.6)
Other postretirement benefits plan expense (income)
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.4	0.3
Interest cost	2.6	2.8
Expected return on plan assets	(3.5)	(3.4)
Amortization, net	(0.7)	(0.7)
Net pension income	$ (1.2)	$ (1.0)