Business Segments (Segment Information) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Total Assets	$ 10,330,856		$ 10,330,856		$ 10,620,852
External Revenues	630,700	$ 421,865	1,560,328	$ 1,637,135
Income (Loss)	137,309	(266,608)	(156,555)	(1,099,710)
Continuing operations
Segment Reporting Information [Line Items]
Total Assets	10,329,900		10,329,900
External Revenues	630,700	421,900	1,560,300	1,637,100
Income (Loss)	138,000	(265,800)	(156,000)	(1,092,800)
Discontinued operations, net of tax
Segment Reporting Information [Line Items]
Total Assets	1,000		1,000
External Revenues	0	0	0	0
Income (Loss)	(700)	(800)	(600)	(6,900)
Corporate
Segment Reporting Information [Line Items]
Total Assets	1,237,800		1,237,800
External Revenues	(59,100)	(5,200)	(493,300)	319,500
Income (Loss)	(98,800)	(72,900)	(577,600)	26,900
Exploration and production | Operating Segments
Segment Reporting Information [Line Items]
Total Assets	9,092,100		9,092,100
External Revenues	689,800	427,100	2,053,600	1,317,600
Income (Loss)	236,800	(192,900)	421,600	(1,119,700)
Exploration and production | Operating Segments | United States
Segment Reporting Information [Line Items]
Total Assets	6,586,300		6,586,300
External Revenues	565,200	330,800	1,704,400	1,070,600
Income (Loss)	168,100	(172,600)	481,800	(1,011,700)
Exploration and production | Operating Segments | Canada
Segment Reporting Information [Line Items]
Total Assets	2,241,200		2,241,200
External Revenues	124,600	96,300	349,200	245,200
Income (Loss)	73,900	(8,600)	(37,700)	(35,000)
Exploration and production | Operating Segments | Other
Segment Reporting Information [Line Items]
Total Assets	264,600		264,600
External Revenues	0	0	0	1,800
Income (Loss)	$ (5,200)	$ (11,700)	$ (22,500)	$ (73,000)