Business Segments (Segment Information) (Details) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended			9 Months Ended		12 Months Ended
		Jun. 30, 2018	Jun. 30, 2017		Jun. 30, 2018	Jun. 30, 2017		Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total Assets		$ 9,945,449			$ 9,945,449					$ 9,860,942
External Revenues		618,194	$ 508,279	[1]	1,203,799	$ 1,184,919	[1]
Net income (loss)		45,518	(17,571)	[1]	213,771	40,893	[1]
Gain (loss) on sale of assets		220	(1,333)		(118)	130,648
Continuing Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		9,923,700			9,923,700
External Revenues		618,200	508,300					$ 1,203,800	$ 1,184,900
Net income (loss)		45,900	(17,400)					214,600	40,100
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Total Assets		21,700			21,700
Net income (loss)		(400)	(200)					(800)	800
Corporate [Member]
Segment Reporting Information [Line Items]
Total Assets	[2]	1,597,300			1,597,300
External Revenues	[2]	(37,600)	31,100					(59,300)	68,200
Net income (loss)	[2]	(105,300)	(67,900)					(52,200)	(161,500)
Exploration And Production Segment [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total Assets	[3]	8,326,400			8,326,400
External Revenues	[3]	655,800	477,200					1,263,100	1,116,700
Net income (loss)	[3]	151,200	50,500					266,800	201,600
Exploration And Production Segment [Member] | Operating Segments [Member] | United States [Member]
Segment Reporting Information [Line Items]
Total Assets	[3]	4,799,100			4,799,100
External Revenues	[3]	318,800	212,500					596,900	436,700
Net income (loss)	[3]	72,600	(9,600)					108,700	(10,600)
Exploration And Production Segment [Member] | Operating Segments [Member] | Canada [Member]
Segment Reporting Information [Line Items]
Total Assets	[3],[4]	1,745,600			1,745,600
External Revenues	[3],[4]	108,400	88,200					226,700	306,100
Net income (loss)	[3],[4]	9,700	5,200					34,300	105,800
Exploration And Production Segment [Member] | Operating Segments [Member] | Malaysia [Member]
Segment Reporting Information [Line Items]
Total Assets	[3]	1,621,700			1,621,700
External Revenues	[3]	228,600	176,500					439,500	373,900
Net income (loss)	[3]	83,900	47,700					154,300	106,300
Exploration And Production Segment [Member] | Operating Segments [Member] | Other Regions [Member]
Segment Reporting Information [Line Items]
Total Assets	[3]	160,000			$ 160,000
Net income (loss)	[3]	$ (15,000)	$ 7,200					$ (30,500)	$ 100
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 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 pages 1 and 30.

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