Business Segments (Segment Information) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Total Assets	$ 10,469,351		$ 10,469,351		$ 11,718,504
External Revenues	421,865	$ 817,077	1,637,135	$ 2,191,573
Income (Loss)	(266,608)	1,111,702	(1,099,710)	1,307,713
Assets/revenue/income (loss) from continuing operations
Segment Reporting Information [Line Items]
Total Assets	10,449,700		10,449,700
External Revenues	421,900	817,100	1,637,100	2,191,600
Income (Loss)	(265,800)	158,300	(1,092,800)	280,100
Discontinued operations, net of tax
Segment Reporting Information [Line Items]
Total Assets	19,700		19,700
External Revenues	0	0	0	0
Income (Loss)	(800)	953,400	(6,900)	1,027,600
Corporate
Segment Reporting Information [Line Items]
Total Assets	1,002,100		1,002,100
External Revenues	(5,200)	63,400	319,500	125,600
Income (Loss)	(72,900)	300	26,900	(97,000)
Exploration and production | Operating Segments
Segment Reporting Information [Line Items]
Total Assets	9,447,600		9,447,600
External Revenues	427,100	753,700	1,317,600	2,066,000
Income (Loss)	(192,900)	158,000	(1,119,700)	377,100
Exploration and production | Operating Segments | United States
Segment Reporting Information [Line Items]
Total Assets	7,028,000		7,028,000
External Revenues	330,800	656,800	1,070,600	1,734,300
Income (Loss)	(172,600)	170,800	(1,011,700)	420,000
Exploration and production | Operating Segments | Canada
Segment Reporting Information [Line Items]
Total Assets	2,155,500		2,155,500
External Revenues	96,300	95,000	245,200	323,800
Income (Loss)	(8,600)	(9,100)	(35,000)	(7,500)
Exploration and production | Operating Segments | Other
Segment Reporting Information [Line Items]
Total Assets	264,100		264,100
External Revenues	0	1,900	1,800	7,900
Income (Loss)	$ (11,700)	$ (3,700)	$ (73,000)	$ (35,400)