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Note 9 - Fair Value - Fair Values for Investments (Details) - USD ($)
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
U.S. Treasury trading securities     $ 83,100,805
U.S. Treasury trading securities     6,406    
U.S. Treasury trading securities     (132,191)    
U.S. Treasury securities available for sale     44,354    
U.S. Treasury securities available for sale     245,494    
Realized gain (loss)     50,760    
Unrealized gain (loss) 206,306 113,303  
Reported Value Measurement [Member]          
U.S. Treasury trading securities      
U.S. Treasury securities available for sale 150,346,440   150,346,440    
Estimate of Fair Value Measurement [Member] | Fair Value, Inputs, Level 1 [Member]          
U.S. Treasury trading securities      
U.S. Treasury securities available for sale $ 150,346,440   $ 150,346,440