Property, Plant and Equipment (Aging of Capitalized Exploratory Well Costs) (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2020 USD ($) well project	Sep. 30, 2019 USD ($) well project	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 187,859	$ 280,735	$ 217,326	$ 207,855
No. of Wells (in wells) | well	12	12
No. of Projects (in projects) | project	5	6
Zero to one year
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 8,000	$ 64,711
No. of Wells (in wells) | well	1	5
No. of Projects (in projects) | project	0	5
One to two years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 54,334	$ 63,615
No. of Wells (in wells) | well	5	1
No. of Projects (in projects) | project	5	1
Two to three years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 0	$ 27,500
No. of Wells (in wells) | well	0	1
No. of Projects (in projects) | project	0	0
Three years or more
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 125,525	$ 124,909
No. of Wells (in wells) | well	6	5
No. of Projects (in projects) | project	0	0