Business Segments (Segment Information) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total Assets	$ 11,205,785		$ 11,718,504
External Revenues	1,003,728	$ 630,546
Income (Loss)	(508,702)	72,769
Assets/revenue/income from continuing operations
Segment Reporting Information [Line Items]
Total Assets	11,185,100
External Revenues	1,003,700	630,500
Income (Loss)	(503,800)	23,000
Discontinued operations, net of tax
Segment Reporting Information [Line Items]
Total Assets	20,700
External Revenues	0	0
Income (Loss)	(4,900)	49,800
Corporate
Segment Reporting Information [Line Items]
Total Assets	1,140,500
External Revenues	400,700	0
Income (Loss)	251,400	(72,400)
Exploration and production | Operating Segments
Segment Reporting Information [Line Items]
Total Assets	10,044,600
External Revenues	603,000	630,500
Income (Loss)	(755,200)	95,400
Exploration and production | Operating Segments | United States
Segment Reporting Information [Line Items]
Total Assets	7,619,500
External Revenues	511,500	500,800
Income (Loss)	(696,000)	116,200
Exploration and production | Operating Segments | Canada
Segment Reporting Information [Line Items]
Total Assets	2,154,200
External Revenues	89,700	126,800
Income (Loss)	(6,900)	7,500
Exploration and production | Operating Segments | Other
Segment Reporting Information [Line Items]
Total Assets	270,900
External Revenues	1,800	2,900
Income (Loss)	$ (52,300)	$ (28,300)