Business Segments (Segment Information) (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total Assets		$ 11,460,697		$ 11,493,812
External Revenues		430,295	$ 921,747
Segment income (loss)		(198,802)	(14,441)
Continuing Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		11,423,500
External Revenues		430,300	921,700
Segment income (loss)		(199,500)	3,500
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		37,200
Segment income (loss)		700	(17,900)
Corporate [Member]
Segment Reporting Information [Line Items]
Total Assets		719,700
External Revenues		1,200	43,600
Segment income (loss)		(42,700)	(15,200)
Exploration and production [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	10,703,800
External Revenues	[1]	429,100	878,100
Segment income (loss)	[1]	(156,800)	18,700
Exploration and production [Member] | Operating Segments [Member] | United States [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	5,587,100
External Revenues	[1]	174,700	280,100
Segment income (loss)	[1]	(65,600)	(93,900)
Exploration and production [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	2,537,100
External Revenues	[1]	106,100	152,300
Segment income (loss)	[1]	(87,300)	(38,500)
Exploration and production [Member] | Operating Segments [Member] | Malaysia [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	2,436,000
External Revenues	[1]	148,200	445,700
Segment income (loss)	[1]	22,300	223,100
Exploration and production [Member] | Operating Segments [Member] | Other Regions [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	143,600
External Revenues	[1]	100
Segment income (loss)	[1]	$ (26,200)	$ (72,000)

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