Quarterly report pursuant to Section 13 or 15(d)

Segment Information (Narrative) (Details)

v3.10.0.1
Segment Information (Narrative) (Details) - segment
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Concentration Risk [Line Items]        
Number of reportable operating segments     2  
Sales Revenue, Net [Member] | Glencore U.K. [Member] | Gabon Segment [Member]        
Concentration Risk [Line Items]        
Concentration Risk, Percentage 100.00% 100.00% 100.00% 100.00%