Accounting for Derivative Instruments and Hedging Activities - Derivative Activity for Instruments Classified as Qualifying Cash Flow Hedges (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Derivative Instruments Gain Loss [Line Items]
Fair Value Asset/(Liability)	$ 0	$ 0.1	$ 0	$ 0.1
Amount of Gain/(Loss) on Derivatives Recognized in OCI	(0.2)	0.7	(0.2)	0.4
Amount of Gain/(Loss) Reclassified From AOCI Into Income	(0.3)	(0.1)	(0.6)	(0.7)
Diesel Fuel Derivatives [Member]
Derivative Instruments Gain Loss [Line Items]
Fair Value Asset/(Liability)	0	0.1	0	0.1
Amount of Gain/(Loss) on Derivatives Recognized in OCI	(0.2)	0.7	(0.2)	0.4
Amount of Gain/(Loss) Reclassified From AOCI Into Income	0	0.1	(0.1)	0
Interest Rate Swap [Member]
Derivative Instruments Gain Loss [Line Items]
Fair Value Asset/(Liability)	0	0	0	0
Amount of Gain/(Loss) on Derivatives Recognized in OCI			0	0
Amount of Gain/(Loss) Reclassified From AOCI Into Income			$ (0.5)	$ (0.7)