Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies (Major Customers) (Details)

v3.8.0.1
Summary of Significant Accounting Policies (Major Customers) (Details) - Sales Revenue, Goods, Net [Member] - Customer Concentration Risk [Member]
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Concentration Risk [Line Items]      
Concentration risk, percentage 100.00% 100.00% 100.00%
Plains Marketing, L.P.      
Concentration Risk [Line Items]      
Concentration risk, percentage 29.00% 16.00% 19.00%
Enterprise Crude Oil, LLC      
Concentration Risk [Line Items]      
Concentration risk, percentage 18.00% 43.00% 42.00%
Rio Energy International, Inc.      
Concentration Risk [Line Items]      
Concentration risk, percentage 17.00% 0.00% 0.00%
Shell Trading Company      
Concentration Risk [Line Items]      
Concentration risk, percentage 9.00% 18.00% 4.00%
Permian Transport and Trading      
Concentration Risk [Line Items]      
Concentration risk, percentage 0.00% 0.00% 15.00%
Other      
Concentration Risk [Line Items]      
Concentration risk, percentage 27.00% 23.00% 20.00%