Segment Reporting (Total Assets for Reportable Segments) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Assets	$ 14,279,411	$ 13,737,695
Eastern [Member]
Segment Reporting Information [Line Items]
Assets	3,060,723	3,099,283
Southern [Member]
Segment Reporting Information [Line Items]
Assets	2,971,182	2,990,247
Western [Member]
Segment Reporting Information [Line Items]
Assets	1,712,098	1,718,015
Central [Member]
Segment Reporting Information [Line Items]
Assets	1,875,151	1,885,468
Canada [Member]
Segment Reporting Information [Line Items]
Assets	2,265,718	2,490,291
Exploration and Production [Member]
Segment Reporting Information [Line Items]
Assets	950,019	962,202
Corporate [Member]
Segment Reporting Information [Line Items]
Assets	$ 1,444,520	$ 592,189