Derivative Instruments and Hedging Activities - Location and Fair Value Amounts of Derivative Instruments Reported in Balance Sheet (Detail) (Commodity Contracts [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Asset Derivatives
Fair Value of Derivative Instruments, Assets	$ 141,366	$ 5,927
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities		8,223
Current liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities		7,753
Long-term liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities		470
Current Asset [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	127,033	4,549
Long-term assets [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	$ 14,333	$ 1,378