Business Segments (Segment Information) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Segment Reporting Information [Line Items]
Total Assets	$ 10,541,737		$ 10,304,940
External Revenues	552,961	$ 379,985
Income (Loss)	(65,486)	(266,824)
Continuing operations
Segment Reporting Information [Line Items]
Total Assets	10,541,000
External Revenues	553,000	380,000
Income (Loss)	(64,900)	(267,000)
Discontinued operations, net of tax
Segment Reporting Information [Line Items]
Total Assets	700
External Revenues	0	0
Income (Loss)	(600)	200
Corporate
Segment Reporting Information [Line Items]
Total Assets	1,157,100
External Revenues	(320,500)	(214,300)
Income (Loss)	(296,300)	(254,800)
Exploration and production | Operating Segments
Segment Reporting Information [Line Items]
Total Assets	9,383,900
External Revenues	873,500	594,300
Income (Loss)	231,400	(12,200)
Exploration and production | Operating Segments | United States
Segment Reporting Information [Line Items]
Total Assets	6,832,000
External Revenues	707,400	490,300
Income (Loss)	252,900	119,000
Exploration and production | Operating Segments | Canada
Segment Reporting Information [Line Items]
Total Assets	2,309,800
External Revenues	166,100	104,000
Income (Loss)	22,700	(124,300)
Exploration and production | Operating Segments | Other
Segment Reporting Information [Line Items]
Total Assets	242,100
External Revenues	0	0
Income (Loss)	$ (44,200)	$ (6,900)