Components of Net Periodic Pension Cost for Defined Pension Plan and Other Postretirement Benefit Plan (Detail) (Pension Benefits, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Benefits
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost - benefits earned during the period	$ 1.6	$ 0.7	$ 0.1
Interest cost	35.2	34.3	32.6
Expected return on plan assets	(38.7)	(37.1)	(35.1)
Recognized net actuarial loss	12.8	15.0	16.8
Amortization of transition asset			(1.6)
Amortization of prior service cost	0.1	0.1	0.1
Net periodic pension cost	$ 11.0	$ 13.0	$ 12.9