Exploratory Costs (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) well	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) well	Jun. 30, 2014 USD ($)	Dec. 31, 2014 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	$ 115,209	$ 109,968	$ 119,439	$ 57,600
Additions pending determination of proved reserves	189,460	200,347	419,521	365,189
Reclassifications due to determination of proved reserves	(204,743)	(142,666)	(439,034)	(255,140)
Exploratory well costs charged to expense	0	(1,191)	0	(1,191)
Capitalized exploratory well costs at end of period	99,926	$ 166,458	99,926	$ 166,458
Capitalized Costs, Oil and Gas Producing Activities, Gross [Abstract]
Exploratory wells in progress (drilling rig not released)	20,424		20,424		$ 18,781
Capitalized exploratory well costs capitalized for a period of one year or less	79,502		79,502		100,658
Total capitalized exploratory well costs	$ 99,926		$ 99,926		$ 119,439
Number of wells in process of drilling | well	48		48