Business Segments (Segment Information) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Total Assets	$ 10,284,344		$ 10,620,852
External Revenues	379,985	$ 1,003,728
Income (Loss)	(266,824)	(508,702)
Assets/revenue/income (loss) from continuing operations
Segment Reporting Information [Line Items]
Total Assets	10,283,500
External Revenues	380,000	1,003,700
Income (Loss)	(267,000)	(503,800)
Discontinued operations, net of tax
Segment Reporting Information [Line Items]
Total Assets	800
External Revenues	0	0
Income (Loss)	200	(4,900)
Corporate
Segment Reporting Information [Line Items]
Total Assets	917,400
External Revenues	(214,300)	400,700
Income (Loss)	(254,800)	251,400
Exploration and production | Operating Segments
Segment Reporting Information [Line Items]
Total Assets	9,366,100
External Revenues	594,300	603,000
Income (Loss)	(12,200)	(755,200)
Exploration and production | Operating Segments | United States
Segment Reporting Information [Line Items]
Total Assets	6,737,800
External Revenues	490,300	511,500
Income (Loss)	119,000	(696,000)
Exploration and production | Operating Segments | Canada
Segment Reporting Information [Line Items]
Total Assets	2,364,000
External Revenues	104,000	89,700
Income (Loss)	(124,300)	(6,900)
Exploration and production | Operating Segments | Other
Segment Reporting Information [Line Items]
Total Assets	264,300
External Revenues	0	1,800
Income (Loss)	$ (6,900)	$ (52,300)