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Note 8 - Long-term Debt (Details Textual)
12 Months Ended
Apr. 29, 2016
USD ($)
ft²
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Apr. 01, 2016
USD ($)
Line of Credit [Member]        
Line of Credit Facility, Maximum Borrowing Capacity   $ 2,050,000    
Long-term Line of Credit, Total   $ 0 $ 0  
Line of Credit [Member] | London Interbank Offered Rate (LIBOR) [Member]        
Debt Instrument, Basis Spread on Variable Rate   1.80%    
Loan [Member]        
Debt Instrument, Periodic Payment, Total $ 18,000      
Debt Instrument, Interest Rate, Stated Percentage 2.60%      
Debt Instrument, Maturity Date Apr. 01, 2041      
Debt Issuance Costs, Gross $ 139,000      
Debt Instrument, Interest Rate, Effective Percentage 3.23%      
Debt Service Coverage Ratio 1.25      
Amortization of Debt Issuance Costs   $ 12,000 $ 12,000  
Debt Issuance Costs, Noncurrent, Net, Total   96,000    
Debt Issuance Costs, Current, Net, Total   11,000    
Amortization of Debt Issuance Costs Annual Expected Amount Over Remainder of Debt Term   $ 10,000    
Debt Instrument Remaining Term   5 years    
Loan [Member] | Building [Member]        
Area of Real Estate Property | ft² 27,300      
Duluth Economic Development Authority [Member]        
Tax Exempt Industrial Revenue Bonds Amount Sold to Bank       $ 3,415,000