Derivative Instruments and Hedging Activities - Location and Fair Value Amounts of Derivative Instruments Reported in Balance Sheet (Detail) (Commodity Contracts [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Asset Derivatives
Fair Value of Derivative Instruments, Assets	$ 21.7	$ 5.9
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	0.2	8.3
Current liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	0.2	7.8
Long-term liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	0	0.5
Current Asset [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	15.4	4.5
Long-term assets [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	$ 6.3	$ 1.4