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Segment Reporting (Total Assets for Reportable Segments) (Details) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Segment Reporting Information [Line Items]    
Assets $ 11,910,275 $ 11,103,925
Southern [Member]    
Segment Reporting Information [Line Items]    
Assets 2,726,600 2,869,841
Western [Member]    
Segment Reporting Information [Line Items]    
Assets 1,560,023 1,516,870
Eastern [Member]    
Segment Reporting Information [Line Items]    
Assets 1,999,928 1,519,576
Canada [Member]    
Segment Reporting Information [Line Items]    
Assets 2,697,750 2,554,324
Central [Member]    
Segment Reporting Information [Line Items]    
Assets 1,312,209 1,302,900
Exploration and Production [Member]    
Segment Reporting Information [Line Items]    
Assets 989,081 1,068,086
Corporate [Member]    
Segment Reporting Information [Line Items]    
Assets $ 624,684 $ 272,328