Supplementary Oil and Natural Gas Disclosures (Schedule of Estimated Net Proved Reserves, Changes in Estimated Net Proved Reserves and Proved Developed Reserves) (Detail)	6 Months Ended	12 Months Ended
	Jun. 30, 2014 MBoe MMcf MBbls	Jun. 30, 2015 MBoe MMcf MBbls	Dec. 31, 2013 MBoe MMcf MBbls	Dec. 31, 2012 MBoe MMcf MBbls
Crude Oil (Mbbls) [Member]
Estimated proved reserves, beginning balance	51,517	60,832	47,442	27,301
Acquisitions	11,640		366	16,430
Sales	(9,214)		(1,415)
Extensions, discoveries and other additions	418	2,415	7,354	6,388
Revisions	386	(8,734)	3,952	1,128
Production	(3,129)	(6,676)	(6,182)	(3,805)
Estimated proved reserves, ending balance	60,832	38,623	51,517	47,442
Proved Developed Reserves (Volume)	45,232	30,804	39,439
Proved Undeveloped Reserve Volume	15,600	7,819	12,078	9,534
Gas (Mmcf) [Member]
Estimated proved reserves, beginning balance | MMcf	173,584	172,081	179,939	58,785
Acquisitions | MMcf	32,731		209	115,876
Sales | MMcf	(13,240)		(916)
Extensions, discoveries and other additions | MMcf	5,352	4,945	20,247	10,241
Revisions | MMcf	(34,791)	(36,237)	(10,128)	4,033
Production | MMcf	(4,795)	(15,963)	(15,767)	(8,996)
Estimated proved reserves, ending balance | MMcf	172,081	111,586	173,584	179,939
Proved Developed Reserves (Volume) | MMcf	101,361	77,562	107,687
Proved Undeveloped Reserve Volume | MMcf	70,720	34,024	65,897	59,252
Oil Equivalent (Mboe) [Member]
Estimated proved reserves, beginning balance (Energy) | MBoe	80,448	89,512	77,432	37,099
Acquisitions (Energy) | MBoe	17,095		401	35,742
Sales (Energy) | MBoe	(11,421)		(1,568)
Extensions and discoveries (Energy) | MBoe	1,310	3,239	10,729	8,095
Revisions (Energy) | MBoe	(5,413)	(14,774)	2,264	1,800
Production (Energy) | MBoe	(3,928)	(9,337)	(8,810)	(5,304)
Estimated proved reserves, ending balance (Energy) | MBoe	89,512	57,219	80,448	77,432
Proved Developed Reserves (Energy) | MBoe	62,126	43,731	57,387	58,022
Proved Undeveloped Reserves (Energy) | MBoe	27,386	13,490	23,061	19,409