Derivative Instruments and Hedging Activities - 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, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total	$ 136,097	$ (26,945)	$ 41,209
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net, Total	526	20,289	30,326
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net, Total	5,721	(2,090)	3,428
Commodity Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain Recognized in Other Comprehensive Income (Loss), Effective Portion, Net, Total	136,097	(26,945)	41,209
Derivative income, 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	5,721	(2,090)	3,428
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	$ 526	$ 20,289	$ 30,326