Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1.6	$ 1.8	$ 4.8	$ 4.7
Interest cost	8.6	8.7	25.8	26.1
Expected return on plan assets	(11.6)	(11.1)	(34.9)	(33.3)
Amortization of prior service cost	0.2	0.2	0.6	0.6
Recognized actuarial loss	3.0	2.3	9.0	6.8
Total net periodic benefit cost	1.8	1.9	5.3	4.9
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.1	0.4	0.3
Interest cost	0.4	0.4	1.2	1.3
Expected return on plan assets	(0.4)	(0.3)	(1.2)	(1.1)
Amortization of prior service cost		(0.1)	(0.1)	(0.2)
Recognized actuarial loss	0.3	0.3	0.9	0.9
Total net periodic benefit cost	$ 0.4	$ 0.4	$ 1.2	$ 1.2