Quarterly report pursuant to Section 13 or 15(d)

Dispositions (Narrative) (Details)

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Dispositions (Narrative) (Details) - Joint Operating With Republic Of Angola [Member] - USD ($)
$ in Millions
1 Months Ended
Nov. 30, 2006
Sep. 30, 2018
Dec. 31, 2017
Business Acquisition [Line Items]      
Joint operation agreement related to third party in working interest percentage 40.00%    
Additional joint operation agreement related to third party in working interest percentage 10.00%    
Stipulated payment for each exploration well, under PSA terms $ 10.0    
Drilling commitment, net $ 5.0    
Liability for the penalty   $ 15.0 $ 15.0