Before Tax Effect of Derivative Instruments in Statement of Operations (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Decrease/increase in oil revenue owing to effective hedging contracts	$ 3,520	$ 8,546	$ 32,706
Decrease/increase in gas revenue owing to effective hedging contracts	$ 16,769	$ 21,780	$ 19,432