Location and Fair Value Amounts of Derivative Instruments Reported in Balance Sheet (Detail) (Commodity Contracts, USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Asset Derivatives
Fair Value of Derivative Instruments, Assets	$ 48,854	$ 47,720
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	(1,679)	(11,937)
Current liabilities
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	(149)	(11,122)
Long-term liabilities
Liability Derivatives
Fair Value of Derivative Instruments, Liabilities	(1,530)	(815)
Current Asset
Asset Derivatives
Fair Value of Derivative Instruments, Assets	39,655	25,177
Long-term assets
Asset Derivatives
Fair Value of Derivative Instruments, Assets	$ 9,199	$ 22,543