Business Segments (Segment Information) (Details) - USD ($) $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018		Jun. 30, 2019	Jun. 30, 2018		Dec. 31, 2018	[1]
Segment Reporting Information [Line Items]
Total Assets	$ 13,536,024			$ 13,536,024			$ 11,052,587
External Revenues	709,049	$ 389,580	[2]	1,300,053	$ 764,370	[2]
Income (Loss)	123,242	45,519	[2]	196,011	213,772	[2],[3]
Assets/revenue/income from continuing operations
Segment Reporting Information [Line Items]
Total Assets	11,672,200			11,672,200
External Revenues	709,000	389,600		1,300,100	764,400
Income (Loss)	98,700	(25,200)		121,700	65,400
Discontinued operations, net of tax
Segment Reporting Information [Line Items]
Total Assets	1,863,800			1,863,800
External Revenues	0	0		0	0
Income (Loss)	24,500	70,700		74,300	148,400
Corporate
Segment Reporting Information [Line Items]
Total Assets	1,053,400			1,053,400
External Revenues	62,100	(37,600)		62,200	(59,200)
Income (Loss)	(25,000)	(92,500)		(97,400)	(47,100)
Exploration and production ¹ | Operating Segments
Segment Reporting Information [Line Items]
Total Assets	10,618,800			10,618,800
External Revenues	646,900	427,200		1,237,900	823,600
Income (Loss)	123,700	67,300		219,100	112,500
Exploration and production ¹ | Operating Segments | United States
Segment Reporting Information [Line Items]
Total Assets	8,089,600			8,089,600
External Revenues	549,000	318,800		1,018,200	596,900
Income (Loss)	133,000	72,500		249,200	108,700
Exploration and production ¹ | Operating Segments | Canada
Segment Reporting Information [Line Items]
Total Assets	2,271,600			2,271,600
External Revenues	94,800	108,400		213,700	226,700
Income (Loss)	(5,900)	9,900		1,600	34,300
Exploration and production ¹ | Operating Segments | Other
Segment Reporting Information [Line Items]
Total Assets	257,600			257,600
External Revenues	3,100	0		6,000	0
Income (Loss)	$ (3,400)	$ (15,100)		$ (31,700)	$ (30,500)

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

[2] Reclassified to conform to current presentation (see Note A).

[3] Reclassified to conform to current presentation (See Note A).