Property, Plant and Equipment (Aging of Capitalized Exploratory Well Costs) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2020 USD ($) well project	Jun. 30, 2019 USD ($) project well	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 180,135	$ 245,017	$ 217,326	$ 207,855
No. of Wells (in wells) | well	11	10
No. of Projects (in projects) | project	5	4
Zero to one year
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 24,429	$ 33,125
No. of Wells (in wells) | well	3	3
No. of Projects (in projects) | project	3	2
One to two years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 30,691	$ 61,293
No. of Wells (in wells) | well	2	1
No. of Projects (in projects) | project	2	1
Two to three years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 0	$ 27,266
No. of Wells (in wells) | well	0	1
No. of Projects (in projects) | project	0	1
Three years or more
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 125,015	$ 123,333
No. of Wells (in wells) | well	6	5
No. of Projects (in projects) | project	0	0