Retirement Plans - Components of Net Pension and Other Postretirement Benefit Plan (OPEB) Income (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension expense (income)
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 0.2	$ 0.2
Interest cost	8.4	7.8
Expected return on plan assets	(11.6)	(12.6)
Amortization, net	1.5	2.0
Settlements	0.0	(0.3)
Net pension income	(1.5)	(2.3)
OPEB expense (income)
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.0	0.4
Interest cost	2.6	2.6
Expected return on plan assets	(3.3)	(3.5)
Amortization, net	(1.8)	(0.7)
Net pension income	$ (2.5)	$ (1.2)