Business Segments (Segment Information) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Mar. 31, 2014		Dec. 31, 2014
Segment Reporting Information [Line Items]
Total Assets	$ 15,160,837				$ 16,723,738	[1]
External Revenues	921,747		1,286,400
Income (Loss)	(14,441)		155,253
Continuing Operations [Member]
Segment Reporting Information [Line Items]
Total Assets	14,782,700
External Revenues	921,700		1,286,400
Income (Loss)	3,500		169,300
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Total Assets	378,100
Income (Loss)	(17,900)		(14,000)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total Assets	1,473,400
External Revenues	43,600		10,400
Income (Loss)	(15,200)		(41,300)
Exploration and production [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Assets	13,309,300	[2]
External Revenues	878,100	[2]	1,276,000	[2]
Income (Loss)	18,700	[2]	210,600	[2]
Exploration and production [Member] | Operating Segments [Member] | United States [Member]
Segment Reporting Information [Line Items]
Total Assets	5,780,700	[2]
External Revenues	280,100	[2]	485,500	[2]
Income (Loss)	(93,900)	[2]	103,100	[2]
Exploration and production [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total Assets	3,436,000	[2]
External Revenues	152,300	[2]	297,700	[2]
Income (Loss)	(38,400)	[2]	67,600	[2]
Exploration and production [Member] | Operating Segments [Member] | Malaysia [Member]
Segment Reporting Information [Line Items]
Total Assets	3,971,300
External Revenues	445,700		492,800
Income (Loss)	223,100		162,300
Exploration and production [Member] | Operating Segments [Member] | Other Regions [Member]
Segment Reporting Information [Line Items]
Total Assets	121,300	[2]
Income (Loss)	$ (72,100)	[2]	$ (122,400)	[2]

[1] Reclassified to conform to current presentation.

[2] Additional details about results of oil and gas operations are presented in the table on page 24.