Supplemental Oil and Gas Reserve Information - Net Proved, Proved Developed and Proved Undeveloped Reserves (Detail)	12 Months Ended
	Dec. 31, 2013 MBbls MBoe MMcf	Dec. 31, 2013 MMcf MBoe MBbls	Dec. 31, 2012 MMcf MBbls MBoe	Dec. 31, 2012 MBbls MBoe MMcf	Dec. 31, 2011 MMcf MBbls MBoe	Dec. 31, 2011 MBbls MBoe MMcf
Estimated Net Proved, Proved Developed and Proved Undeveloped Reserves [Roll Forward]
Proved reserves (Volume), beginning balance	128,090	128,090	57,900	57,900	19,379	19,379
Proved reserves (Energy), beginning balance	143,345	143,345	78,715	78,715	39,780	39,780
Revisions of previous estimates (Volume)	(3,390)	(10,411)	(8,495)	(567)	(159)	(262)
Revisions of previous estimates (Energy)	(5,125)	(5,125)	(1,983)	(1,983)	(288)	(288)
Extensions, discoveries and other additions (Volume)	40,784	31,856	45,759	66,245	40,220	36,464
Extensions, discoveries and other additions (Energy)	46,094	46,094	73,871	73,871	43,168	43,168
Sales of reserves in place (Volume)			0	0	(518)	(56)
Sales of reserves in place (Energy)			0	0	(142)	(142)
Purchases of reserves in place (Volume)	37,459	49,631	512	881	65	100
Purchases of reserves in place (Energy)	45,731	45,731	966	966	111	111
Production (Volume)	(11,133)	(7,450)	(4,146)	(7,533)	(1,087)	(3,732)
Production (Energy)	(12,375)	(12,375)	(8,224)	(8,224)	(3,914)	(3,914)
Net proved reserves (Volume), closing balance	175,979	175,979	128,090	128,090	57,900	57,900
Net proved reserves (Energy), closing balance	227,920	227,920	143,345	143,345	78,715	78,715
Proved developed reserves (Volume)	92,170	92,170	62,602	62,602	24,535	24,535
Proved developed reserves (Energy)	122,136	122,136	70,051	70,051	35,839	35,839
Proved undeveloped reserve (Volume)	83,809	83,809	65,488	65,488	33,365	33,365
Proved undeveloped reserves (Energy)	105,784	105,784	73,294	73,294	42,876	42,876