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Segment Reporting (Total Assets for Reportable Segments) (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Segment Reporting Information [Line Items]    
Assets $ 12,192,066 $ 12,014,681
Southern [Member]    
Segment Reporting Information [Line Items]    
Assets 2,778,123 2,718,296
Western [Member]    
Segment Reporting Information [Line Items]    
Assets 1,583,561 1,573,955
Eastern [Member]    
Segment Reporting Information [Line Items]    
Assets 2,348,438 2,024,527
Canada [Member]    
Segment Reporting Information [Line Items]    
Assets 2,562,659 2,677,557
Central [Member]    
Segment Reporting Information [Line Items]    
Assets 1,452,607 1,297,118
Exploration and Production [Member]    
Segment Reporting Information [Line Items]    
Assets 974,049 981,980
Corporate [Member]    
Segment Reporting Information [Line Items]    
Assets $ 492,629 $ 741,248