Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended		12 Months Ended
	Mar. 31, 2021	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Well costs		$ 186,623	$ 187,859	$ 181,616	$ 217,326
Exploratory well costs capitalized more than one year		183,300
Impairment of oil and gas Properties	$ 171,300	171,296	1,206,284	$ 1,206,300
Proceeds from sale of oil and gas property and equipment	$ 267,700
Vietnam
Property, Plant and Equipment [Line Items]
Exploratory well costs capitalized more than one year		92,300
United States
Property, Plant and Equipment [Line Items]
Exploratory well costs capitalized more than one year		45,000
Impairment of oil and gas Properties		0	1,152,515
Brunei
Property, Plant and Equipment [Line Items]
Exploratory well costs capitalized more than one year		25,900
MEXICO
Property, Plant and Equipment [Line Items]
Exploratory well costs capitalized more than one year		15,300
Canada
Property, Plant and Equipment [Line Items]
Exploratory well costs capitalized more than one year		4,800
Impairment of oil and gas Properties		$ 171,296	$ 0