Accounting for Derivative Instruments and Hedging Activities - Derivative Activity for Instruments Classified as Qualifying Cash Flow Hedges (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative Instruments, Gain (Loss) [Line Items]
Fair Value Asset/(Liability)	$ (0.9)	$ (0.3)	$ 1.8
Amount of Gain/(Loss) Recognized in OCI	(4.6)	1.2	0.6
Amount of Gain/(Loss) Reclassified From AOCI Into Income	(0.4)	2.0	(2.5)
Interest rate contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair Value Asset/(Liability)	0	0	0
Amount of Gain/(Loss) Recognized in OCI	(4.9)	0	0
Amount of Gain/(Loss) Reclassified From AOCI Into Income	(0.8)	(0.7)	(1.7)
Diesel Fuel Derivatives [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair Value Asset/(Liability)	(0.9)	(0.3)	1.8
Amount of Gain/(Loss) Recognized in OCI	0.3	1.2	0.6
Amount of Gain/(Loss) Reclassified From AOCI Into Income	$ 0.4	$ 2.7	$ (0.8)