Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1.6	$ 1.6	$ 3.2	$ 3.2
Interest cost	8.3	8.6	16.6	17.2
Expected return on plan assets	(11.6)	(11.7)	(23.2)	(23.3)
Amortization of prior service cost	0.2	0.2	0.4	0.4
Recognized actuarial loss	4.0	3.0	8.0	6.0
Total net periodic benefit cost	2.5	1.7	5.0	3.5
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.2	0.2	0.3
Interest cost	0.3	0.4	0.6	0.8
Expected return on plan assets	(0.4)	(0.4)	(0.8)	(0.8)
Amortization of prior service cost	(0.1)	(0.1)	(0.2)	(0.1)
Recognized actuarial loss	0.3	0.3	0.6	0.6
Total net periodic benefit cost	$ 0.2	$ 0.4	$ 0.4	$ 0.8