Employee Postretirement Benefits - Schedule of Change in Plan Assets (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Change in plan assets
Fair value of plan assets at prior measurement date	$ 467.6
Employer contributions	35.5
Fair value of plan assets at measurement date	529.1
Pension Benefits [Member]
Change in plan assets
Fair value of plan assets at prior measurement date	467.6	479.7
Actual return on plan assets	57.9	21.8
Employer contributions	36.8	3.7
Plan participants' contributions	0	0
Settlements	0	(2.4)
Net benefits paid	(33.2)	(35.2)
Fair value of plan assets at measurement date	529.1	467.6
Other Postretirement Benefits [Member]
Change in plan assets
Fair value of plan assets at prior measurement date	0	0
Actual return on plan assets	0	0
Employer contributions	9.8	11.2
Plan participants' contributions	3.7	3.9
Settlements	0	0
Net benefits paid	(14.5)	(16.2)
Fair value of plan assets at measurement date	$ 0	$ 0