Business Segments (Segment Information) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021	Dec. 31, 2021
Segment Reporting Information [Line Items]
Total Assets	$ 10,572,211		$ 10,572,211		$ 10,304,940
External Revenues	1,101,057	$ 549,643	1,654,018	$ 929,628
Income (Loss)	409,504	(27,040)	344,018	(293,864)
Continuing operations
Segment Reporting Information [Line Items]
Total Assets	10,571,200		10,571,200
External Revenues	1,101,100	549,600	1,654,000	929,600
Income (Loss)	410,400	(26,900)	345,500	(294,000)
Discontinued operations, net of tax
Segment Reporting Information [Line Items]
Total Assets	1,000		1,000
External Revenues	0	0	0	0
Income (Loss)	(900)	(100)	(1,500)	100
Corporate
Segment Reporting Information [Line Items]
Total Assets	1,041,300		1,041,300
External Revenues	(97,200)	(219,900)	(417,800)	(434,200)
Income (Loss)	(124,800)	(223,900)	(421,100)	(478,800)
Exploration and production | Operating Segments
Segment Reporting Information [Line Items]
Total Assets	9,529,900		9,529,900
External Revenues	1,198,300	769,500	2,071,800	1,363,800
Income (Loss)	535,200	197,000	766,600	184,800
Exploration and production | Operating Segments | United States
Segment Reporting Information [Line Items]
Total Assets	7,034,900		7,034,900
External Revenues	978,000	648,900	1,685,400	1,139,200
Income (Loss)	491,500	194,700	744,400	313,700
Exploration and production | Operating Segments | Canada
Segment Reporting Information [Line Items]
Total Assets	2,250,100		2,250,100
External Revenues	206,600	120,600	372,700	224,600
Income (Loss)	47,200	12,700	69,900	(111,600)
Exploration and production | Operating Segments | Other
Segment Reporting Information [Line Items]
Total Assets	244,900		244,900
External Revenues	13,700	0	13,700	0
Income (Loss)	$ (3,500)	$ (10,400)	$ (47,700)	$ (17,300)