Annual report pursuant to Section 13 and 15(d)

Significant Accounting Policies - Percentage of Revenue by Major Customers (Details)

v3.6.0.2
Significant Accounting Policies - Percentage of Revenue by Major Customers (Details) - Sales Revenue Net - Customer Concentration Risk
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Shell Trading (US) Co.      
Entity Wide Revenue Major Customer [Line Items]      
Percentage of receipts 43.00% 50.00% 47.00%
Vitol Inc.      
Entity Wide Revenue Major Customer [Line Items]      
Percentage of receipts 20.00%    
J. P. Morgan      
Entity Wide Revenue Major Customer [Line Items]      
Percentage of receipts   14.00%