Location and Fair Value Amounts of Derivative Instruments Reported in Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
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	4,549
Long-term assets [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	1,378
Commodity Contracts [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	5,927	48,854
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	8,223	1,679
Commodity Contracts [Member] | Current liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	7,753	149
Commodity Contracts [Member] | Long-term liabilities [Member]
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	470	1,530
Commodity Contracts [Member] | Current Asset [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	4,549	39,655
Commodity Contracts [Member] | Long-term assets [Member]
Asset Derivatives
Fair Value of Derivative Instruments, Assets	$ 1,378	$ 9,199