Business Segments (Segment Information) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018		Dec. 31, 2018	[1]
Segment Reporting Information [Line Items]
Total Assets		$ 11,983,117			$ 11,052,587
External Revenues		591,004	$ 374,790	[1]
Income (loss)		40,182	168,253	[1]
Continuing Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		10,103,500
External Revenues		591,000	374,800
Income (loss)		23,000	90,600
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		1,879,600
Income (loss)		49,800	77,700
Corporate [Member]
Segment Reporting Information [Line Items]
Total Assets		1,010,000
External Revenues			(21,600)
Income (loss)		(72,400)	45,400
Exploration And Production Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Assets	[2]	9,093,500
External Revenues	[2]	591,000	396,400
Income (loss)	[2]	95,400	45,200
Exploration And Production Segment [Member] | Operating Segments [Member] | United States [Member]
Segment Reporting Information [Line Items]
Total Assets	[2]	6,615,500
External Revenues	[2]	469,200	278,100
Income (loss)	[2]	116,200	36,200
Exploration And Production Segment [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total Assets	[2]	2,252,800
External Revenues	[2]	118,900	118,300
Income (loss)	[2]	7,500	24,400
Exploration And Production Segment [Member] | Operating Segments [Member] | Other Regions [Member]
Segment Reporting Information [Line Items]
Total Assets	[2]	225,200
External Revenues	[2]	2,900
Income (loss)	[2]	$ (28,300)	$ (15,400)

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

[2] Additional details about results of oil and gas operations are presented in the tables on pages 29.