Suspended Well Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (Decrease) in Capitalized Exploratory Well Costs that are Pending Determination of Proved Reserves [Roll Forward]
Beginning balance	$ 19,846	$ 11,952	$ 43,589
Additions to capitalized exploratory well costs pending the determination of proved reserves	49,446	19,846	11,952
Divestitures	0	0	(809)
Reclassification to wells, facilities, and equipment based on the determination of proved reserves	(19,846)	(11,952)	(18,485)
Capitalized exploratory well costs charged to expense	0	0	(24,295)
Ending balance	49,446	$ 19,846	$ 11,952
Exploratory well costs capitalized for more than one year	$ 0