Effect of Derivative Instruments in Statement of Operations (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total	$ 3,918	$ 20,290	$ 292
Oil
Derivative Instruments, Gain (Loss) [Line Items]
Decrease/increase in oil revenue owing to effective hedging contracts	8,546	32,706	29,047
Natural Gas
Derivative Instruments, Gain (Loss) [Line Items]
Increase in gas revenue owing to effective hedging contracts	$ 21,780	$ 19,432	$ 38,678