Asset Impairment (Summary of Impairments) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment			$ 1,292,308	$ 416,801	$ 13,906
Assets impairments from discontinued operations			0	1,936	29,794
Total asset impairments			1,292,308	418,737	43,700
Central Basin Platform
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment			484,848	0	0
Delaware Basin
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	$ 90,600		607,303	90,594	0
Midland Basin
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment		$ 25,800	0	25,776	0
San Juan Basin properties
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment			133,055	230,315	0
Permian Basin Unproved Leasehold Properties
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment	$ 55,100		29,168	64,361	13,906
San Juan Basin unproved leasehold properties
Impaired Long-Lived Assets Held and Used [Line Items]
Asset impairment			37,934	5,755	0
North Louisiana/East Texas oil and natural gas properties
Impaired Long-Lived Assets Held and Used [Line Items]
Assets impairments from discontinued operations			$ 0	$ 1,936	$ 29,794