Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost - benefits earned during the period	$ 0.4	$ 0.4
Interest cost	7.9	8.8
Expected return on plan assets	(8.9)	(9.7)
Recognized net actuarial loss	3.7	3.2
Net periodic pension cost (benefit)	3.1	2.7
Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Interest cost	0.5	0.5
Recognized net actuarial loss	0.1
Amortization of prior service cost	(0.9)	(0.9)
Net periodic pension cost (benefit)	$ (0.3)	$ (0.4)