Asset Impairment - Summary of Impairments Recognized (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Mar. 31, 2014	Jun. 30, 2015	Jun. 30, 2014
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	$ 60,413	$ 1,342		$ 66,996	$ 2,588
Impairment of oil and gas properties held-for-sale	0	0		0	1,667
Asset impairment charges, including discontinued operations	60,413	1,342		66,996	4,255
Permian Basin and San Juan Basin [Member]
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	8,900			11,200
Permian Basin
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	8,939	1,339		10,944	2,587
San Juan Basin
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	0	3		248	1
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	$ 51,474	$ 0		$ 55,804	$ 0