Exploratory Costs (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2015 USD ($) well	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) well	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($) well
Increase (Decrease) in Capitalized Exploratory Well Costs that are Pending Determination of Proved Reserves [Roll Forward]
Capitalized exploratory well costs at beginning of period	$ 99,926	$ 166,458	$ 119,439	$ 57,600	$ 57,600
Additions pending determination of proved reserves	113,734	258,724	533,255	623,913
Reclassifications due to determination of proved reserves	(82,325)	(258,123)	(521,359)	(513,263)
Exploratory well costs charged to expense	0	(1,217)	0	(2,408)
Capitalized exploratory well costs at end of period	131,335	$ 165,842	131,335	$ 165,842	119,439
Capitalized Costs, Oil and Gas Producing Activities, Gross [Abstract]
Exploratory wells in progress (drilling rig not released)	14,161		14,161		18,781
Capitalized exploratory well costs capitalized for a period of one year or less	117,174		117,174		100,658
Total capitalized exploratory well costs	$ 131,335		$ 131,335		$ 119,439
Number of wells capitalized | well			0		0
Number of wells in process of drilling | well	55		55
Permian Basin
Capitalized Costs, Oil and Gas Producing Activities, Gross [Abstract]
Number of wells in process of drilling | well	47		47
San Juan Basin
Capitalized Costs, Oil and Gas Producing Activities, Gross [Abstract]
Number of wells in process of drilling | well	8		8