Business Segments (Segment Information) (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total Assets		$ 10,192,759		$ 10,192,759		$ 10,295,860
External Revenues		498,319	$ 485,546	1,683,238	$ 1,329,688
Net income (loss)		(65,893)	(16,176)	(25,000)	(212,048)
Continuing Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		10,169,600		10,169,600
External Revenues		498,300	485,500	1,683,200	1,329,700
Net income (loss)		(66,300)	(14,600)	(26,200)	(211,200)
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		23,200		23,200
Net income (loss)		400	(1,600)	1,200	(800)
Corporate [Member]
Segment Reporting Information [Line Items]
Total Assets		1,124,200		1,124,200
External Revenues			(100)	4,000	3,500
Net income (loss)		(99,900)	(39,600)	(302,800)	(111,700)
Exploration and production [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	9,045,400		9,045,400
External Revenues	[1]	498,300	485,600	1,679,200	1,326,200
Net income (loss)	[1]	33,600	25,000	276,600	(99,500)
Exploration and production [Member] | Operating Segments [Member] | United States [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	5,439,100		5,439,100
External Revenues	[1]	195,900	201,800	696,700	520,200
Net income (loss)	[1]	(19,900)	(27,100)	11,000	(158,500)
Exploration and production [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	1,711,100		1,711,100
External Revenues	[1]	81,900	80,900	388,100	264,400
Net income (loss)	[1]	(3,200)	(4,800)	102,600	(36,900)
Exploration and production [Member] | Operating Segments [Member] | Malaysia [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	1,755,300		1,755,300
External Revenues	[1]	220,500	202,700	594,400	541,400
Net income (loss)	[1]	67,700	65,000	173,900	135,100
Exploration and production [Member] | Operating Segments [Member] | Other Regions [Member]
Segment Reporting Information [Line Items]
Total Assets	[1]	139,900		139,900
External Revenues	[1]		200		200
Net income (loss)	[1]	$ (11,000)	$ (8,100)	$ (10,900)	$ (39,200)

[1] Additional details about results of oil and gas operations are presented in the tables on pages 29 and 30.