Business Segments (Segment Information) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	[1]
Segment Reporting Information [Line Items]
Total Assets		$ 15,149,964		$ 15,149,964		$ 16,723,738
External Revenues		738,290	$ 1,349,021	1,660,037	$ 2,635,421
Income (Loss)		(73,834)	129,412	(88,275)	284,665
Continuing Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		14,870,200		14,870,200
External Revenues		738,300	1,349,000	1,660,000	2,635,400
Income (Loss)		(89,000)	142,700	(85,500)	312,000
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		279,800		279,800
Income (Loss)		15,200	(13,300)	(2,800)	(27,300)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total Assets		1,424,500		1,424,500
External Revenues		1,100	(3,900)	44,700	6,500
Income (Loss)		(37,800)	(58,100)	(53,000)	(99,400)
Exploration and production [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Assets	[2]	13,445,700		13,445,700
External Revenues	[2]	737,200	1,352,900	1,615,300	2,628,900
Income (Loss)	[2]	(51,200)	200,800	(32,500)	411,400
Exploration and production [Member] | Operating Segments [Member] | United States [Member]
Segment Reporting Information [Line Items]
Total Assets	[2]	6,015,000		6,015,000
External Revenues	[2]	339,800	507,300	619,900	992,800
Income (Loss)	[2]	(16,500)	101,700	(110,400)	204,800
Exploration and production [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total Assets	[2]	3,477,000		3,477,000
External Revenues	[2]	152,900	262,800	305,200	560,500
Income (Loss)	[2]	(32,200)	52,900	(70,700)	120,500
Exploration and production [Member] | Operating Segments [Member] | Malaysia [Member]
Segment Reporting Information [Line Items]
Total Assets	[2]	3,838,900		3,838,900
External Revenues	[2]	244,500	583,000	690,200	1,075,800
Income (Loss)	[2]	27,600	172,300	250,700	334,600
Exploration and production [Member] | Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Total Assets	[2]	114,800		114,800
External Revenues	[2]		(200)		(200)
Income (Loss)	[2]	$ (30,100)	$ (126,100)	$ (102,100)	$ (248,500)

[1] Reclassified to conform to current presentation.

[2] Additional details about results of oil and gas operations are presented in the table on pages 25 and 26.