Property, Plant and Equipment Aging of Capitalized Exploratory Well Costs - (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($) well project	Jun. 30, 2018 USD ($) well project	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 245,017	$ 185,596	$ 207,855	$ 155,103
No. of Wells (in wells) | well	10	12
No. of Projects (in projects) | project	4	5
Zero to one year
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 33,125	$ 34,779
No. of Wells (in wells) | well	3	2
No. of Projects (in projects) | project	2	2
One to two years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 61,293	$ 35,934
No. of Wells (in wells) | well	1	2
No. of Projects (in projects) | project	1	1
Two to three years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 27,266	$ 50,272
No. of Wells (in wells) | well	1	2
No. of Projects (in projects) | project	1	2
Three years or more
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 123,333	$ 64,611
No. of Wells (in wells) | well	5	6
No. of Projects (in projects) | project	0	0