Before Tax Effect of Derivative Instruments in Statement of Operations (Detail) (Cash Flow Hedging [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total	$ (26,945)		$ 41,209		$ 43,089
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net, Total	20,289	[1]	30,326	[1]	(13,274)	[1]
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net, Total	(2,090)		3,428		1,418
Commodity Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total	(26,945)		41,209		43,089
Derivative income (expense), net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net, Total	(2,090)		3,428		1,418
Operating revenue - oil/gas production [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net, Total	$ 20,289	[1]	$ 30,326	[1]	$ (13,274)	[1]

[1] For the year ended December 31, 2013, effective hedging contracts increased oil revenue by $3,520 and increased gas revenue by $16,769. For the year ended December 31, 2012, effective hedging contracts increased oil revenue by $8,546 and increased gas revenue by $21,780. For the year ended December 31, 2011, effective hedging contracts decreased oil revenue by $32,706 and increased gas revenue by $19,432.