Suspended Exploratory Well Costs - Suspended Exploratory Well Costs (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) Project	Dec. 31, 2016 USD ($) Project	Dec. 31, 2015 USD ($) Project
Extractive Industries [Abstract]
Balance at beginning of period	$ 7,412	$ 4,161	$ 2,996
Additions to capitalized exploratory well costs pending the determination of proved reserves	1,388	9,128	1,165
Reclassifications to wells, facilities and equipment based on determination of proved reserves	0	(5,877)	0
Capitalized exploratory well costs charged to expense	(8,800)	0	0
Balance at end of period	0	7,412	4,161
Less exploratory well costs that have been capitalized for a period of one year or less	0	(7,412)	(1,165)
Capitalized exploratory well costs that have been capitalized for a period greater than one year	$ 0	$ 0	$ 2,996
Number of projects that have exploratory well costs that have been capitalized for a period greater than one year | Project	0	0	1