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	Mar. 31, 2015	Dec. 31, 2014
Asset Derivatives
Fair Value of Derivative Instruments, Assets	$ 127.0	$ 141.3
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	0	0
Current Liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	0	0
Long-Term Liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	0	0
Current Assets [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	113.0	127.0
Long-Term Assets [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	$ 14.0	$ 14.3