Business Segments (Segment Information) (Details) - USD ($) $ in Thousands		3 Months Ended			9 Months Ended			12 Months Ended
		Sep. 30, 2018	Sep. 30, 2017		Sep. 30, 2018	Sep. 30, 2017		Dec. 31, 2017
Segment Reporting Information [Line Items]
Total Assets		$ 10,026,620			$ 10,026,620			$ 9,860,942
External Revenues		674,797	$ 498,319	[1]	1,878,596	$ 1,683,238	[1]
Net income (loss)		93,944	(65,893)	[1]	307,715	(25,000)	[1]
Gain (loss) on sale of assets		124	117		6	130,765
Continuing Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		10,005,500			10,005,500
External Revenues		674,800	498,300		1,878,600	1,683,200
Net income (loss)		95,700	(66,300)		310,400	(26,200)
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		21,100			21,100
Net income (loss)		(1,800)	400		(2,700)	1,200
Corporate [Member]
Segment Reporting Information [Line Items]
Total Assets	[2]	1,654,900			1,654,900
External Revenues	[2]	(2,100)	(13,500)		(61,400)	54,400
Net income (loss)	[2]	(63,800)	(108,600)		(116,200)	(270,000)
Exploration And Production Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Assets	[3]	8,350,600			8,350,600
External Revenues	[3]	676,900	511,800		1,940,000	1,628,800
Net income (loss)	[3]	159,500	42,300		426,600	243,800
Exploration And Production Segment [Member] | Operating Segments [Member] | United States [Member]
Segment Reporting Information [Line Items]
Total Assets	[3]	4,772,200			4,772,200
External Revenues	[3]	348,700	209,400		945,600	646,300
Net income (loss)	[3]	91,600	(11,200)		200,300	(21,800)
Exploration And Production Segment [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total Assets	[3],[4]	1,790,300			1,790,300
External Revenues	[3],[4]	107,100	81,900		333,800	388,100
Net income (loss)	[3],[4]	12,500	(3,200)		46,700	102,600
Exploration And Production Segment [Member] | Operating Segments [Member] | Malaysia [Member]
Segment Reporting Information [Line Items]
Total Assets	[3]	1,604,700			1,604,700
External Revenues	[3]	201,200	220,500		640,700	594,400
Net income (loss)	[3]	54,100	67,700		208,400	173,900
Exploration And Production Segment [Member] | Operating Segments [Member] | Other Regions [Member]
Segment Reporting Information [Line Items]
Total Assets	[3]	183,400			183,400
External Revenues	[3]	19,900			19,900
Net income (loss)	[3]	$ 1,300	$ (11,000)		$ (28,800)	(10,900)
Western Canada [Member]
Segment Reporting Information [Line Items]
Gain (loss) on sale of assets						$ 132,400		$ 132,400

[1] Reclassified to conform to current presentation (see Note B).

[2] In 2018, the Company reported realized and unrealized gains and losses on crude oil contracts in the Corporate segment (previously reported in the Exploration and production business) to reflect how segments are currently evaluated, how resources are allocated and how risk is managed by the Company. The 2017 amounts have been reclassified to reflect comparable disclosure.

[3] Additional details about results of oil and gas operations are presented in the tables on pages 30 and 31.

[4] Revenue for the nine months ended September 30, 2017 includes a pretax gain of $132.4 million related to the sale of Seal heavy oil assets in Canada.