Employee Postretirement Benefits - Schedule of Amounts Recognized in Accumulated Other Comprehensive Income (Loss) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)	$ 52.7	$ 50.3
Prior service cost (credit)	0.4	0.5
Transition obligation	0	0
Amount recognized	53.1	50.8
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)	(17.2)	(20.0)
Prior service cost (credit)	(0.7)	(0.8)
Transition obligation	0	0.4
Amount recognized	$ (17.9)	$ (20.4)