Business Segments (Segment Information) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total Assets		$ 9,914,632		$ 9,914,632		$ 11,493,812
External Revenues		437,462	$ 738,290	867,757	$ 1,660,037
Net income (loss)		2,930	(73,834)	(195,872)	(88,275)
Continuing Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		9,882,500		9,882,500
External Revenues		437,500	738,300	867,800	1,660,000
Net income (loss)		2,900	(89,000)	(196,600)	(85,500)
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		32,100		32,100
Net income (loss)			15,200	700	(2,800)
Corporate [Member]
Segment Reporting Information [Line Items]
Total Assets		559,400		559,400
External Revenues		26,100	1,100	27,200	44,700
Net income (loss)		(29,300)	(37,800)	(72,200)	(53,000)
Exploration and production [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	9,323,100		9,323,100
External Revenues	[1]	411,400	737,200	840,600	1,615,300
Net income (loss)	[1]	32,200	(51,200)	(124,400)	(32,500)
Exploration and production [Member] | Operating Segments [Member] | United States [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	5,479,200		5,479,200
External Revenues	[1]	143,600	339,800	318,300	619,900
Net income (loss)	[1]	(65,700)	(16,500)	(131,400)	(110,400)
Exploration and production [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	1,580,300		1,580,300
External Revenues	[1]	77,400	152,900	183,500	305,200
Net income (loss)	[1]	55,300	(32,200)	(31,900)	(70,700)
Exploration and production [Member] | Operating Segments [Member] | Malaysia [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	2,130,400		2,130,400
External Revenues	[1]	190,500	244,500	338,800	690,200
Net income (loss)	[1]	47,700	27,600	70,100	250,700
Exploration and production [Member] | Operating Segments [Member] | Other Regions [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	133,200		133,200
External Revenues	[1]	(100)
Net income (loss)	[1]	$ (5,100)	$ (30,100)	$ (31,200)	$ (102,100)

[1] Additional details about results of oil and gas operations are presented in the tables on pages 26 and 27.