Components of Net Periodic Pension Cost for Defined Pension Plan and Other Postretirement Benefit Plan (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
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	$ 1.2	$ 0.4
Interest cost	8.8	9.2	26.4	25.0
Expected return on plan assets	(9.6)	(9.9)	(29.0)	(27.0)
Recognized net actuarial loss	3.2	3.8	9.6	11.3
Amortization of prior service cost	0.1		0.1
Net periodic cost (benefit)	2.9	3.5	8.3	9.7
Postretirement Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost - benefits earned during the period		0.1	0.1	0.1
Interest cost	0.6	0.6	1.6	1.8
Recognized net actuarial loss	0.1	0.1	0.2	0.3
Amortization of prior service cost	(1.0)	(1.3)	(2.9)	(3.9)
Net periodic cost (benefit)	$ (0.3)	$ (0.5)	$ (1.0)	$ (1.7)