Pension and Postretirement Benefit Plans - Company's Share of Net Periodic Cost Components (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1.7	$ 1.6
Interest cost	2.9	2.9
Expected return on plan assets	(4.2)	(3.7)
Amortization of:
Negative prior service cost	0.0	0.0
Net actuarial loss	2.0	2.3
Net periodic cost (benefit)	2.4	3.1
Postretirement [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0.0	0.0
Interest cost	0.2	0.2
Expected return on plan assets	0.0	0.0
Amortization of:
Negative prior service cost	(1.4)	(1.4)
Net actuarial loss	0.1	0.1
Net periodic cost (benefit)	$ (1.1)	$ (1.1)