Asset Impairment - Summary of Impairments Recognized (Details) - USD ($) $ in Thousands	3 Months Ended				9 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Sep. 30, 2014	Mar. 31, 2014	Sep. 30, 2015	Sep. 30, 2014
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	$ 399,394		$ 178,912		$ 466,390	$ 181,500
Impairment of oil and gas properties held-for-sale	0		0		0	1,667
Asset impairment charges, including discontinued operations	399,394		178,912		466,390	183,167
Permian Basin and San Juan Basin
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment					20,300
Permian Basin
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment			31,000
Central Basin
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	371,593		0		423,067	0
Delaware Basin
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	18,653		0		22,983	0
Midland Basin
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	0		25,776		0	25,776
San Juan Basin
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	0		142,172		0	142,172
Permian Basin Unproved Leasehold Properties
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	9,148		5,231		20,092	7,818
San Juan Basin Unproved Leasehold Properties [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	0		5,733		248	5,734
North Louisiana and East Texas Natural Gas and Oil Properties
Impaired Long-Lived Assets Held and Used [Line Items]
Impairment of oil and gas properties held-for-sale	0		$ 0	$ 1,700	0	$ 1,667
Oil | Permian Basin
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	$ 390,246				$ 446,100
Oil | Central Basin
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		$ 51,500