Exploratory Costs - Exploratory Well Costs (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) well	Mar. 31, 2017 USD ($)	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	$ 132,200	$ 164,996	$ 164,996
Additions pending determination of proved reserves	168,753	164,966
Reclassifications due to determination of proved reserves	(158,946)	(161,697)
Capitalized exploratory well costs at end of period	142,007	$ 168,265	132,200
Capitalized Costs, Oil and Gas Producing Activities, Gross [Abstract]
Exploratory wells in progress (drilling rig not released)	13,644		10,879
Capitalized exploratory well costs capitalized for a period of one year or less	128,363		121,321
Total capitalized exploratory well costs	$ 142,007		$ 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	38