Business Segments (Segment Information) (Details) - USD ($) $ in Thousands	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018		Sep. 30, 2019	Sep. 30, 2018		Dec. 31, 2018	[1]
Segment Reporting Information [Line Items]
Total Assets	$ 11,783,667			$ 11,783,667			$ 11,052,587
External Revenues	817,077	$ 490,511	[2]	2,191,573	$ 1,287,763	[2]
Income (Loss)	1,111,702	93,943	[2]	1,307,713	307,715	[2],[3]
Assets/revenue/income from continuing operations
Segment Reporting Information [Line Items]
Total Assets	11,766,500			11,766,500
External Revenues	817,100	490,500		2,191,600	1,287,800
Income (Loss)	158,300	56,100		280,100	121,500
Discontinued operations, net of tax
Segment Reporting Information [Line Items]
Total Assets	17,200			17,200
External Revenues	0	0		0	0
Income (Loss)	953,400	37,800		1,027,600	186,200
Corporate
Segment Reporting Information [Line Items]
Total Assets	1,151,200			1,151,200
External Revenues	63,400	(2,100)		125,600	(61,400)
Income (Loss)	300	(49,300)		(97,000)	(96,700)
Exploration and production | Operating Segments
Segment Reporting Information [Line Items]
Total Assets	10,615,300			10,615,300
External Revenues	753,700	492,600		2,066,000	1,349,200
Income (Loss)	158,000	105,400		377,100	218,200
Exploration and production | Operating Segments | United States
Segment Reporting Information [Line Items]
Total Assets	8,149,500			8,149,500
External Revenues	656,800	357,800		1,734,300	971,700
Income (Loss)	170,800	91,600		420,000	200,300
Exploration and production | Operating Segments | Canada
Segment Reporting Information [Line Items]
Total Assets	2,212,500			2,212,500
External Revenues	95,000	114,900		323,800	357,600
Income (Loss)	(9,100)	12,500		(7,500)	46,700
Exploration and production | Operating Segments | Other
Segment Reporting Information [Line Items]
Total Assets	253,300			253,300
External Revenues	1,900	19,900		7,900	19,900
Income (Loss)	$ (3,700)	$ 1,300		$ (35,400)	$ (28,800)

[1] Reclassified to conform to current presentation (see Note A).

[2] Reclassified to conform to current presentation (see Note A).

[3] Reclassified to conform to current presentation (See Note A).