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Divestitures (Narrative) (Details) - USD ($)
$ in Millions
6 Months Ended
Apr. 03, 2018
Jun. 30, 2018
Jun. 30, 2017
Noncontrolling Interest [Line Items]      
Proceeds from non-core oil and natural gas asset sales, net of related expenses   $ 22.4 $ 17.8
Ownership interest in Superior Pipeline Company, L.L.C. [Member]      
Noncontrolling Interest [Line Items]      
Ownership interest sold 50.00%    
Proceeds from sale of non-controlling interest $ 300.0