Pension and Postretirement Benefit Plans - Company's Share of Net Periodic Cost Components (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1.0	$ 1.3	$ 2.0	$ 2.6
Interest cost	2.8	2.6	5.7	5.3
Expected return on plan assets	(4.1)	(4.5)	(8.3)	(9.0)
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	0.0	0.0	0.0	0.0
Amortization of:
Defined Benefit Plan, Amortization of Gain (Loss)	1.5	2.0	3.0	4.1
Net periodic cost (benefit)	1.2	1.4	2.4	3.0
Postretirement [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0.0	0.0	0.0	0.0
Interest cost	0.1	0.1	0.3	0.3
Expected return on plan assets	0.0	0.0	0.0	0.0
Defined Benefit Plan, Amortization of Prior Service Cost (Credit)	(1.3)	(1.3)	(2.7)	(2.7)
Amortization of:
Defined Benefit Plan, Amortization of Gain (Loss)	0.0	0.0	0.1	0.1
Net periodic cost (benefit)	$ (1.2)	$ (1.2)	$ (2.3)	$ (2.3)