Derivative Instruments and Hedging Activities - Location and Fair Value Amounts of Derivative Instruments Reported in Balance Sheet (Detail) - Designated as Hedging Instrument [Member] - Commodity Contracts [Member] - USD ($) $ in Millions	Sep. 30, 2015	Dec. 31, 2014
Asset Derivatives
Fair Value of Derivative Instruments, Assets	$ 67.1	$ 141.3
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	0.2	0.0
Current Liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	0.0	0.0
Long-Term Liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	0.2	0.0
Current Assets [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	61.4	127.0
Long-Term Assets [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	$ 5.7	$ 14.3