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Note 1 - Significant Accounting Policies - Percentage of Revenue by Major Customers (Details) - Revenue from Contract with Customer Benchmark [Member] - Customer Concentration Risk [Member]
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Shell Trading (US) Co. [Member]      
Concentration risk percentage 11.00% 30.00% 46.00%
BP Products North America [Member]      
Concentration risk percentage 40.00% 20.00% [1]
Vitol Inc. [Member]      
Concentration risk percentage 12.00% 14.00% 15.00%
[1] Less than 10%