Pension and Postretirement Benefit Plans - Company's Share of Net Periodic Cost Components (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1.2	$ 1.0
Interest cost	2.4	2.9
Expected return on plan assets	(4.4)	(4.2)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	0.0	0.0
Amortization of:
Defined Benefit Plan, Amortization of Gain (Loss)	2.3	1.5
Net periodic cost (benefit)	1.5	1.2
Postretirement [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0.0	0.0
Interest cost	0.1	0.2
Expected return on plan assets	0.0	0.0
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	(1.4)	(1.4)
Amortization of:
Defined Benefit Plan, Amortization of Gain (Loss)	0.1	0.1
Net periodic cost (benefit)	$ (1.2)	$ (1.1)