Property, Plant and Equipment (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended		12 Months Ended
	Mar. 31, 2021	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Well costs		$ 197,537	$ 180,135	$ 181,616	$ 217,326
Exploratory well costs capitalized more than one year		183,600
Impairment of oil and gas Properties	$ 171,300	171,296	987,146	$ 987,100
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		91,500
United States
Property, Plant and Equipment [Line Items]
Exploratory well costs capitalized more than one year		46,200
Impairment of oil and gas Properties		0	947,437
Brunei
Property, Plant and Equipment [Line Items]
Exploratory well costs capitalized more than one year		25,700
MEXICO
Property, Plant and Equipment [Line Items]
Exploratory well costs capitalized more than one year		15,400
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