Business Segments (Segment Information) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total Assets		$ 10,394,015		$ 10,394,015		$ 11,493,812
External Revenues		500,533	$ 714,949	1,368,290	$ 2,374,986
Net income (loss)		(16,176)	(1,595,426)	(212,048)	(1,683,701)
Continuing Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		10,364,000		10,364,000
External Revenues		500,500	714,900	1,368,300	2,375,000
Net income (loss)		(14,600)	(1,587,100)	(211,200)	(1,672,500)
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		30,000		30,000
Net income (loss)		(1,600)	(8,300)	(800)	(11,200)
Corporate [Member]
Segment Reporting Information [Line Items]
Total Assets		1,051,500		1,051,500
External Revenues		14,900	49,300	42,100	94,000
Net income (loss)		(39,600)	9,000	(111,700)	(44,000)
Exploration and production [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	9,312,500		9,312,500
External Revenues	[1]	485,600	665,600	1,326,200	2,281,000
Net income (loss)	[1]	25,000	(1,596,100)	(99,500)	(1,628,500)
Exploration and production [Member] | Operating Segments [Member] | United States [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	5,448,900		5,448,900
External Revenues	[1]	201,800	335,100	520,200	955,000
Net income (loss)	[1]	(27,100)	(107,800)	(158,500)	(218,100)
Exploration and production [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	1,557,300		1,557,300
External Revenues	[1]	80,900	123,200	264,400	428,400
Net income (loss)	[1]	(4,800)	(507,000)	(36,900)	(577,800)
Exploration and production [Member] | Operating Segments [Member] | Malaysia [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	2,186,100		2,186,100
External Revenues	[1]	202,700	207,300	541,400	897,600
Net income (loss)	[1]	65,000	(952,700)	135,100	(701,900)
Exploration and production [Member] | Operating Segments [Member] | Other Regions [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	120,200		120,200
External Revenues	[1]	200		200
Net income (loss)	[1]	$ (8,100)	$ (28,600)	$ (39,200)	$ (130,700)

[1] Additional details about results of oil and gas operations are presented in the tables on pages 27 and 28.