Summary of Significant Accounting Policies - Net Changes in Capitalized Exploratory Well Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Increase (Decrease) in Capitalized Exploratory Well Costs that are Pending Determination of Proved Reserves [Roll Forward]
Beginning of period	$ 34,522	$ 123,215	$ 40,424
Exploratory well cost additions (pending determination of proved reserves)	51,995	336,344	346,814
Exploratory well cost reclassifications (successful determination of proved reserves)	(84,228)	(425,037)	(264,023)
Exploratory well dry hole costs (unsuccessful in adding proved reserves)	0	0	0
End of period	$ 2,289	$ 34,522	$ 123,215