Pension and Postretirement Benefit Plans - Company's Share of Net Periodic Cost Components (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Pension [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 2.0	$ 1.3
Interest cost	2.8	2.8
Expected return on plan assets	(3.4)	(3.2)
Amortization of:
Prior service benefits	0	0
Net actuarial loss	2.7	1.4
Net periodic cost (benefit)	4.1	2.3
Postretirement [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0	0
Interest cost	0.2	0.3
Expected return on plan assets	0	0
Amortization of:
Prior service benefits	(1.4)	(1.4)
Net actuarial loss	0.2	0.2
Net periodic cost (benefit)	$ (1.0)	$ (0.9)