Employee Postretirement Benefits - Schedule of Net Periodic Benefit Cost (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 17.0	$ 16.0	$ 16.2
Interest cost	30.1	30.9	33.2
Expected return on plan assets	(37.1)	(38.4)	(36.3)
Actuarial loss	15.3	11.3	12.4
Prior service (benefit) cost	(0.4)	(0.4)	(0.4)
Transition obligation	0	0	0
Settlement loss	0	0.9	1.6
Net periodic benefit cost	24.9	20.3	26.7
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	2.4	2.1	3.2
Interest cost	10.1	11.0	10.9
Expected return on plan assets	0	0	0
Actuarial loss	0.1	0.1	0
Prior service (benefit) cost	0.8	0.8	0.8
Transition obligation	1.8	2.3	2.3
Settlement loss	0	0	0
Net periodic benefit cost	$ 15.2	$ 16.4	$ 17.2