Employee Postretirement Benefits - Schedule of Obligation and Funded Status (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Benefits [Member]
Change in benefit obligation
Net benefit obligation at prior measurement date	$ 646.4	$ 610.3
Service cost	17.0	16.0	16.2
Interest cost	30.1	30.9	33.2
Plan participants' contributions	0	0
Plan amendments	0	0
Actuarial loss (gain)	54.7	26.8
Gross benefits paid	(33.2)	(35.2)
Settlements	0	(2.4)
Net benefit obligation at measurement date	715.0	646.4	610.3
Other Postretirement Benefits [Member]
Change in benefit obligation
Net benefit obligation at prior measurement date	216.5	222.0
Service cost	2.4	2.1	3.2
Interest cost	10.1	11.0	10.9
Plan participants' contributions	3.7	3.9
Plan amendments	(5.2)	0
Actuarial loss (gain)	16.3	(7.4)
Gross benefits paid	(14.5)	(16.2)
Settlements	0	0
Federal subsidy on benefits paid	1.0	1.1
Net benefit obligation at measurement date	$ 230.3	$ 216.5	$ 222.0