Business Segments (Segment Information) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total Assets		$ 10,136,801		$ 10,136,801		$ 10,295,860
External Revenues		474,497	$ 437,462	1,139,116	$ 867,757
Net income (loss)		(17,571)	2,930	40,893	(195,872)
Continuing Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		10,114,600		10,114,600
External Revenues		474,500	437,500	1,139,100	867,800
Net income (loss)		(17,400)	2,900	40,100	(196,600)
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		22,200		22,200
Net income (loss)		(200)		800	700
Corporate [Member]
Segment Reporting Information [Line Items]
Total Assets		1,229,900		1,229,900
External Revenues		(29,700)	26,100	(41,700)	27,200
Net income (loss)		(85,500)	(29,300)	(203,100)	(72,200)
Exploration and production [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	8,884,700		8,884,700
External Revenues	[1]	504,200	411,400	1,180,800	840,600
Net income (loss)	[1]	68,100	32,200	243,200	(124,400)
Exploration and production [Member] | Operating Segments [Member] | United States [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	5,324,400		5,324,400
External Revenues	[1]	239,500	143,600	500,800	318,300
Net income (loss)	[1]	8,000	(65,700)	31,000	(131,400)
Exploration and production [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	1,629,200		1,629,200
External Revenues	[1]	88,200	77,400	306,100	183,500
Net income (loss)	[1]	5,200	55,300	105,800	(31,900)
Exploration and production [Member] | Operating Segments [Member] | Malaysia [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	1,795,000		1,795,000
External Revenues	[1]	176,500	190,500	373,900	338,800
Net income (loss)	[1]	47,700	47,700	106,300	70,100
Exploration and production [Member] | Operating Segments [Member] | Other Regions [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	136,100		136,100
External Revenues	[1]		(100)
Net income (loss)	[1]	$ 7,200	$ (5,100)	$ 100	$ (31,200)

[1] Additional details about results of oil and gas operations are presented in the tables on pages 27 and 28.