Exploratory Costs - Exploratory Well Costs (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($) well	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) well	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) well
Increase (Decrease) in Capitalized Exploratory Well Costs that are Pending Determination of Proved Reserves [Roll Forward]
Capitalized exploratory well costs at beginning of period	$ 168,265	$ 105,591	$ 164,996	$ 103,588	$ 103,588
Additions pending determination of proved reserves	170,737	78,457	335,703	161,903
Reclassifications due to determination of proved reserves	(197,601)	(146,610)	(359,298)	(228,053)
Capitalized exploratory well costs at end of period	141,401	$ 37,438	141,401	$ 37,438	164,996
Capitalized Costs, Oil and Gas Producing Activities, Gross [Abstract]
Exploratory wells in progress (drilling rig not released)	15,224		15,224		14,531
Capitalized exploratory well costs capitalized for a period of one year or less	126,177		126,177		143,602
Capitalized exploratory well costs for a period greater than one year	0		0		6,863
Total capitalized exploratory well costs	$ 141,401		$ 141,401		$ 164,996
Restructuring Cost and Reserve [Line Items]
Number of wells in process of drilling | well	51		51
Number of wells capitalized | well			0		2