Components of Net Periodic Postretirement Benefit Cost (Detail) (Postretirement Benefits, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 0.1	$ 0.1	$ 0.1
Interest cost	2.1	2.4	2.7
Recognized net actuarial loss	0.2	0.4	0.4
Amortization of prior service cost	(3.8)	(5.3)	(5.9)
Net periodic pension cost	$ (1.4)	$ (2.4)	$ (2.7)