Exploratory Costs (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015 well	Mar. 31, 2014	Dec. 31, 2014
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
Additions pending determination of proved reserves	230,061	164,842
Reclassifications due to determination of proved reserves	(234,291)	(112,474)
Capitalized exploratory well costs at end of period	115,209	109,968
Exploratory wells in progress (drilling rig not released)	13,063		18,781
Capitalized exploratory well costs capitalized for a period of one year or less	102,146		100,658
Total capitalized exploratory well costs	$ 115,209		$ 119,439
Number of wells in process of drilling	41