Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1.3	$ 1.6
Interest cost	7.2	8.3
Expected return on plan assets	(9.8)	(11.6)
Amortization of prior service cost	0.3	0.2
Recognized actuarial loss	4.4	4.0
Total net periodic benefit cost	3.4	2.5
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.1
Interest cost	0.3	0.3
Expected return on plan assets	(0.4)	(0.4)
Amortization of prior service cost	(0.1)	(0.1)
Recognized actuarial loss	0.3	0.3
Total net periodic benefit cost	$ 0.2	$ 0.2