Components of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6.4	$ 6.4	$ 5.3
Interest cost	34.5	34.9	35.1
Expected return on plan assets	(46.6)	(44.8)	(44.8)
Amortization of prior service cost	0.8	0.8	0.8
Recognized actuarial loss	12.0	9.2	8.7
Special termination benefit			0.1
Total net periodic benefit cost	7.1	6.5	5.2
Other Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.6	0.5	0.5
Interest cost	1.6	1.7	1.8
Expected return on plan assets	(1.7)	(1.5)	(1.5)
Amortization of prior service cost	(0.2)	(0.2)	(0.2)
Recognized actuarial loss	1.3	1.2	1.1
Total net periodic benefit cost	$ 1.6	$ 1.7	$ 1.7