Business Segments (Segment Information) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Total Assets	$ 10,604,190		$ 10,604,190		$ 10,620,852
External Revenues	549,643	$ 211,542	929,628	$ 1,215,270
Income (Loss)	(27,040)	(324,400)	(293,864)	(833,102)
Continuing operations
Segment Reporting Information [Line Items]
Total Assets	10,603,000		10,603,000
External Revenues	549,600	211,500	929,600	1,215,300
Income (Loss)	(26,900)	(323,200)	(294,000)	(827,000)
Discontinued operations, net of tax
Segment Reporting Information [Line Items]
Total Assets	1,200		1,200
External Revenues	0	0	0	0
Income (Loss)	(100)	(1,200)	100	(6,100)
Corporate
Segment Reporting Information [Line Items]
Total Assets	1,097,700		1,097,700
External Revenues	(219,900)	(76,000)	(434,200)	324,800
Income (Loss)	(223,900)	(151,600)	(478,800)	99,800
Exploration and production | Operating Segments
Segment Reporting Information [Line Items]
Total Assets	9,505,300		9,505,300
External Revenues	769,500	287,500	1,363,800	890,500
Income (Loss)	197,000	(171,600)	184,800	(926,800)
Exploration and production | Operating Segments | United States
Segment Reporting Information [Line Items]
Total Assets	6,868,200		6,868,200
External Revenues	648,900	228,300	1,139,200	739,800
Income (Loss)	194,700	(143,100)	313,700	(839,100)
Exploration and production | Operating Segments | Canada
Segment Reporting Information [Line Items]
Total Assets	2,366,700		2,366,700
External Revenues	120,600	59,200	224,600	148,900
Income (Loss)	12,700	(19,500)	(111,600)	(26,400)
Exploration and production | Operating Segments | Other
Segment Reporting Information [Line Items]
Total Assets	270,400		270,400
External Revenues	0	0	0	1,800
Income (Loss)	$ (10,400)	$ (9,000)	$ (17,300)	$ (61,300)