Property, Plant and Equipment Net Changes in Capitalized Exploratory Well Costs - (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Property, Plant and Equipment [Abstract]
Beginning balance at January 1	$ 207,855	$ 155,103
Additions pending the determination of proved reserves	50,307	30,493
Capitalized exploratory well costs charged to expense	(13,145)	0
Balance at June 30	$ 245,017	$ 185,596