Exploratory Costs - Exploratory Well Costs (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) well	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) well	Sep. 30, 2017 USD ($)	Dec. 31, 2017 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	$ 128,234	$ 141,401	$ 132,200	$ 164,996	$ 164,996
Additions pending determination of proved reserves	202,743	168,965	578,784	504,668
Reclassifications due to determination of proved reserves	(206,031)	(174,270)	(586,038)	(533,568)
Capitalized exploratory well costs at end of period	124,946	$ 136,096	124,946	$ 136,096	132,200
Capitalized Costs, Oil and Gas Producing Activities, Gross [Abstract]
Exploratory wells in progress (drilling rig not released)	17,578		17,578		10,879
Capitalized exploratory well costs capitalized for a period of one year or less	107,368		107,368		121,321
Total capitalized exploratory well costs	$ 124,946		$ 124,946		$ 132,200
Restructuring Cost and Reserve [Line Items]
Number of wells capitalized | well			0		0
Permian Basin
Restructuring Cost and Reserve [Line Items]
Number of wells in process of drilling | well	49		49