Business Segments (Segment Information) (Details) - USD ($) $ in Thousands		3 Months Ended			12 Months Ended
		Mar. 31, 2018	Mar. 31, 2017		Dec. 31, 2017
Segment Reporting Information [Line Items]
Total Assets		$ 9,938,210			$ 9,860,942
External Revenues		585,605	$ 676,640	[1]
Net income (loss)		168,253	58,464	[1]
Gain (loss) on sale of assets		(339)	131,982
Continuing Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		9,915,500
External Revenues		585,600	676,600
Net income (loss)		168,700	57,500
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		22,700
Net income (loss)		(400)	1,000
Corporate [Member]
Segment Reporting Information [Line Items]
Total Assets	[2]	1,625,700
External Revenues	[2]	(21,600)	37,100
Net income (loss)	[2]	53,100	(93,500)
Exploration And Production Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Assets	[3]	8,289,800
External Revenues	[3]	607,200	639,500
Net income (loss)	[3]	115,600	151,000
Exploration And Production Segment [Member] | Operating Segments [Member] | United States [Member]
Segment Reporting Information [Line Items]
Total Assets	[3]	4,750,000
External Revenues	[3]	278,100	224,200
Net income (loss)	[3]	36,200	(1,100)
Exploration And Production Segment [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total Assets	[3],[4]	1,751,500
External Revenues	[3],[4]	118,300	218,000
Net income (loss)	[3],[4]	24,400	100,600
Exploration And Production Segment [Member] | Operating Segments [Member] | Malaysia [Member]
Segment Reporting Information [Line Items]
Total Assets	[3]	1,624,800
External Revenues	[3]	210,800	197,300
Net income (loss)	[3]	70,400	58,600
Exploration And Production Segment [Member] | Operating Segments [Member] | Other Regions [Member]
Segment Reporting Information [Line Items]
Total Assets	[3]	163,500
Net income (loss)	[3]	$ (15,400)	$ (7,100)
Western Canada [Member]
Segment Reporting Information [Line Items]
Gain (loss) on sale of assets					$ 132,400

[1] Reclassified to conform to current presentation (see Notes A and 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 page 28.

[4] 2017 revenue includes a pretax gain of $132.4 million related to the sale of Seal heavy oil assets in Canada.