Property, Plant and Equipment (Aging of Capitalized Exploratory Well Costs) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2021 USD ($) well project	Sep. 30, 2020 USD ($) well project	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 186,623	$ 187,859	$ 181,616	$ 217,326
No. of Wells (in wells) | well	13	12
No. of Projects (in projects) | project	7	5
Zero to one year
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 3,297	$ 8,000
No. of Wells (in wells) | well	2	1
No. of Projects (in projects) | project	2	0
One to two years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 0	$ 54,334
No. of Wells (in wells) | well	0	5
No. of Projects (in projects) | project	0	5
Two to three years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 53,078	$ 0
No. of Wells (in wells) | well	5	0
No. of Projects (in projects) | project	5	0
Three years or more
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 130,248	$ 125,525
No. of Wells (in wells) | well	6	6
No. of Projects (in projects) | project	0	0