Business Segments (Segment Information) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total Assets		$ 10,271,987		$ 10,295,860
External Revenues		664,619	$ 430,295
Net income (loss)		58,464	(198,802)
Continuing Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		10,246,500
External Revenues		664,600	430,300
Net income (loss)		57,500	(199,500)
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		25,500
Net income (loss)		1,000	700
Corporate [Member]
Segment Reporting Information [Line Items]
Total Assets		1,213,200
External Revenues		(12,000)	1,200
Net income (loss)		(117,600)	(42,700)
Exploration and production [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	9,033,300
External Revenues	[1]	676,600	429,100
Net income (loss)	[1]	175,100	(156,800)
Exploration and production [Member] | Operating Segments [Member] | United States [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	5,332,600
External Revenues	[1]	261,300	174,700
Net income (loss)	[1]	23,000	(65,600)
Exploration and production [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	1,601,100
External Revenues	[1]	218,000	106,100
Net income (loss)	[1]	100,600	(87,300)
Exploration and production [Member] | Operating Segments [Member] | Malaysia [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	1,972,200
External Revenues	[1]	197,300	148,200
Net income (loss)	[1]	58,600	22,300
Exploration and production [Member] | Operating Segments [Member] | Other Regions [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	127,400
External Revenues	[1]		100
Net income (loss)	[1]	$ (7,100)	$ (26,200)

[1] Additional details about results of oil and gas operations are presented in the table on page 25.