Pension and Other Post-Retirement Benefits - Components of Pension and Other Post Retirement Benefit Costs (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Defined Benefit Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1.5	$ 1.4	$ 4.5	$ 4.1
Interest cost	4.9	5.2	14.8	15.6
Expected return on plan assets	(7.4)	(8.4)	(22.2)	(25.0)
Unrecognized prior service credit	(0.3)	(0.3)	(0.8)	(0.8)
Unrecognized net actuarial loss (gain)	2.5	1.8	7.5	5.4
Net costs	1.2	(0.3)	3.8	(0.7)
Retiree Health and Life [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0.1	0.1	0.2	0.2
Interest cost	0.5	0.6	1.5	1.7
Unrecognized prior service credit			(0.1)	(0.1)
Unrecognized net actuarial loss (gain)		(0.1)	(0.1)	(0.2)
Net costs	$ 0.6	$ 0.6	$ 1.5	$ 1.6