Property, Plant and Equipment (Aging of Capitalized Exploratory Well Costs) (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2021 USD ($) well project	Jun. 30, 2020 USD ($) well project	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 197,537	$ 180,135	$ 181,616	$ 217,326
No. of Wells (in wells) | well	14	11
No. of Projects (in projects) | project	8	5
Zero to one year
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 13,881	$ 24,429
No. of Wells (in wells) | well	3	3
No. of Projects (in projects) | project	3	3
One to two years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 23,811	$ 30,691
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 | $	$ 129,283	$ 125,015
No. of Wells (in wells) | well	6	6
No. of Projects (in projects) | project	0	0