Exploratory Well Costs (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Capitalized Exploratory Well Costs [Abstract]
Balance at January 1		$ 0	$ 8,955	$ 17,253
Additions Pending the Determination of Proved Reserves		27,487	6,688	24,640
Reclassifications to Proved Properties		(20,802)	(5,274)	(26,659)
Costs Charged to Expense (1)	[1]	(4,518)	(10,369)	(6,279)
Balance at December 31		$ 2,167	$ 0	$ 8,955

[1] Includes capitalized exploratory well costs charged to either dry hole costs or impairments.