Property, Plant and Equipment Aging of Capitalized Exploratory Well Costs - (Details) $ in Thousands	9 Months Ended
	Sep. 30, 2019 USD ($) well project	Sep. 30, 2018 USD ($) well project	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 280,735	$ 189,928	$ 207,855	$ 155,103
No. of Wells (in wells) | well	12	9
No. of Projects (in projects) | project	6	5
Zero to one year
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 64,711	$ 46,813
No. of Wells (in wells) | well	5	1
No. of Projects (in projects) | project	5	1
One to two years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 63,615	$ 41,051
No. of Wells (in wells) | well	1	3
No. of Projects (in projects) | project	1	2
Two to three years
Projects with Exploratory Well Costs Capitalized for More than One Year [Line Items]
Amount | $	$ 27,500	$ 5,208
No. of Wells (in wells) | well	1	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 | $	$ 124,909	$ 96,856
No. of Wells (in wells) | well	5	4
No. of Projects (in projects) | project	0	1