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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1.3	$ 1.5	$ 3.0	$ 3.1
Interest cost	2.8	2.9	5.7	5.9
Expected return on plan assets	(4.2)	(3.7)	(8.4)	(7.4)
Amortization of:
Negative prior service cost	0.0	0.0	0.0	0.0
Net actuarial loss	1.9	2.3	3.9	4.6
Net periodic cost (benefit)	1.8	3.0	4.2	6.2
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	0.2	0.2	0.4	0.4
Expected return on plan assets	0.0	0.0	0.0	0.0
Amortization of:
Negative prior service cost	(1.4)	(1.4)	(2.8)	(2.8)
Net actuarial loss	0.0	0.0	0.1	0.1
Net periodic cost (benefit)	$ (1.2)	$ (1.2)	$ (2.3)	$ (2.3)