Property, Plant and Equipment (Aging of Capitalized Exploratory Well Costs) (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($) project well	Mar. 31, 2020 USD ($) well project	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 182,401	$ 178,433	$ 181,616	$ 217,326
No. of Wells (in wells) | well	11	11
No. of Projects (in projects) | project	5	5
Zero to one year
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 0	$ 24,637
No. of Wells (in wells) | well	0	3
No. of Projects (in projects) | project	0	3
One to two years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 23,514	$ 31,541
No. of Wells (in wells) | well	3	2
No. of Projects (in projects) | project	3	2
Two to three years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 30,562	$ 0
No. of Wells (in wells) | well	2	0
No. of Projects (in projects) | project	2	0
Three years or more
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 128,325	$ 122,255
No. of Wells (in wells) | well	6	6
No. of Projects (in projects) | project	0	0