Business Segments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013		Dec. 31, 2013
Segment Reporting Information [Line Items]
Total Assets	$ 17,845,766				$ 17,845,766				$ 17,509,484
External Revenues	1,433,037		1,419,496	[1]	4,068,458		4,042,420	[1]
Income (Loss)	245,707		284,809	[1]	530,373		1,048,052	[1]
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total Assets	1,254,400				1,254,400
External Revenues	2,100		53,100		8,600		61,700
Income (Loss)	(40,400)		800	[2]	(139,800)		(78,700)	[2]
Exploration and production [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Assets	15,788,300	[3]			15,788,300	[3]
External Revenues	1,430,900	[3]	1,366,400	[3]	4,059,800	[3]	3,980,700	[3]
Income (Loss)	311,400	[3]	264,200	[2],[3]	722,800	[3]	786,300	[2],[3]
Exploration and production [Member] | Operating Segments [Member] | United States [Member]
Segment Reporting Information [Line Items]
Total Assets	5,620,200	[3]			5,620,200	[3]
External Revenues	667,600	[3]	512,000	[3]	1,660,400	[3]	1,365,100	[3]
Income (Loss)	130,500	[3]	151,300	[2],[3]	335,300	[3]	368,000	[2],[3]
Exploration and production [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total Assets	3,933,000	[3]			3,933,000	[3]
External Revenues	246,900	[3]	316,400	[3]	807,400	[3]	894,000	[3]
Income (Loss)	40,400	[3]	77,300	[2],[3]	160,900	[3]	142,300	[2],[3]
Exploration and production [Member] | Operating Segments [Member] | Malaysia [Member]
Segment Reporting Information [Line Items]
Total Assets	6,100,000	[3]			6,100,000	[3]
External Revenues	516,400	[3]	538,000	[3]	1,592,200	[3]	1,652,700	[3]
Income (Loss)	148,000	[3]	183,800	[2],[3]	482,600	[3]	602,500	[2],[3]
Exploration and production [Member] | Operating Segments [Member] | Other Regions [Member]
Segment Reporting Information [Line Items]
Total Assets	135,100	[3]			135,100	[3]
External Revenues					(200)	[3]	68,900	[3]
Income (Loss)	(7,500)	[3]	(148,200)	[2],[3]	(256,000)	[3]	(326,500)	[2],[3]
Continuing Operations [Member]
Segment Reporting Information [Line Items]
Total Assets	17,042,700				17,042,700
External Revenues	1,433,000		1,419,500		4,068,400		4,042,400
Income (Loss)	271,000		265,000	[2]	583,000		707,600	[2]
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Total Assets	803,100				803,100
Income (Loss)	$ (25,300)		$ 19,800	[2]	$ (52,600)		$ 340,400	[2]

[1] Reclassified to conform to current presentation - See Note D.

[2] Reclassified to conform to current presentation.

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