Segment Reporting (Total Assets for Reportable Segments) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Assets	$ 12,024,544	$ 12,014,681
Southern [Member]
Segment Reporting Information [Line Items]
Assets	2,777,535	2,718,296
Western [Member]
Segment Reporting Information [Line Items]
Assets	1,563,670	1,573,955
Eastern [Member]
Segment Reporting Information [Line Items]
Assets	2,339,444	2,024,527
Canada [Member]
Segment Reporting Information [Line Items]
Assets	2,526,702	2,677,557
Central [Member]
Segment Reporting Information [Line Items]
Assets	1,437,434	1,297,118
Exploration and Production [Member]
Segment Reporting Information [Line Items]
Assets	971,047	981,980
Corporate [Member]
Segment Reporting Information [Line Items]
Assets	$ 408,712	$ 741,248