Exploratory Costs (Capitalized Exploratory Well Costs) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) well	Dec. 31, 2014 USD ($) well	Dec. 31, 2013 USD ($)
Increase (Decrease) in Capitalized Exploratory Well Costs that are Pending Determination of Proved Reserves [Roll Forward]
Capitalized exploratory well costs at beginning of period	$ 119,439	$ 57,600	$ 79,791
Additions pending determination of proved reserves	634,908	946,751	421,599
Reclassifications due to determination of proved reserves	(650,759)	(882,254)	(442,909)
Exploratory well costs charged to expense	0	(2,658)	(881)
Capitalized exploratory well costs at end of period	103,588	119,439	$ 57,600
Capitalized Exploratory Well Costs [Abstract]
Exploratory wells in progress (drilling rig not released)	1,760	18,781
Capitalized exploratory well costs for a period of one year or less	101,828	100,658
Total capitalized exploratory well costs	$ 103,588	$ 119,439
Number of wells capitalized for a period greater than one year | well	0	0
Wells in process of drilling | well	40