Business Segments (Segment Information) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total Assets	$ 10,754,030		$ 10,754,030		$ 11,718,504
External Revenues	211,542	$ 743,950	1,215,270	$ 1,374,496
Income (Loss)	(324,400)	123,242	(833,102)	196,011
Assets/revenue/income (loss) from continuing operations
Segment Reporting Information [Line Items]
Total Assets	10,733,600		10,733,600
External Revenues	211,500	744,000	1,215,300	1,374,500
Income (Loss)	(323,200)	98,800	(827,000)	121,700
Discontinued operations, net of tax
Segment Reporting Information [Line Items]
Total Assets	20,400		20,400
External Revenues	0	0	0	0
Income (Loss)	(1,200)	24,400	(6,100)	74,300
Corporate
Segment Reporting Information [Line Items]
Total Assets	915,700		915,700
External Revenues	(76,000)	62,200	324,800	62,100
Income (Loss)	(151,600)	(24,900)	99,800	(97,400)
Exploration and production | Operating Segments
Segment Reporting Information [Line Items]
Total Assets	9,817,900		9,817,900
External Revenues	287,500	681,800	890,500	1,312,400
Income (Loss)	(171,600)	123,700	(926,800)	219,100
Exploration and production | Operating Segments | United States
Segment Reporting Information [Line Items]
Total Assets	7,363,800		7,363,800
External Revenues	228,300	576,700	739,800	1,077,500
Income (Loss)	(143,100)	133,000	(839,100)	249,200
Exploration and production | Operating Segments | Canada
Segment Reporting Information [Line Items]
Total Assets	2,184,900		2,184,900
External Revenues	59,200	102,000	148,900	228,900
Income (Loss)	(19,500)	(5,900)	(26,400)	1,600
Exploration and production | Operating Segments | Other
Segment Reporting Information [Line Items]
Total Assets	269,200		269,200
External Revenues	0	3,100	1,800	6,000
Income (Loss)	$ (9,000)	$ (3,400)	$ (61,300)	$ (31,700)