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Note 5 - Credit Facilities (Details Textual) - USD ($)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Jun. 30, 2016
Interest Expense, Debt, Total $ 1,000 $ 1,000 $ 17,000  
Line of Credit Maturing February 2022 [Member]        
Line of Credit Facility, Maximum Borrowing Capacity $ 20,000,000      
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage 0.10%      
Line of Credit Facility, Remaining Borrowing Capacity $ 19,900,000 19,700,000    
Letters of Credit Outstanding, Amount $ 100,000 300,000    
Line of Credit Maturing February 2022 [Member] | London Interbank Offered Rate (LIBOR) [Member]        
Debt Instrument, Basis Spread on Variable Rate 0.75%      
Line of Credit Maturing May 2021 [Member]        
Line of Credit Facility, Maximum Borrowing Capacity $ 8,000,000      
Line of Credit Facility, Remaining Borrowing Capacity 3,600,000 2,400,000    
Letters of Credit Outstanding, Amount $ 4,400,000 5,600,000    
Line of Credit Maturing May 2021 [Member] | London Interbank Offered Rate (LIBOR) [Member]        
Debt Instrument, Basis Spread on Variable Rate 0.75%      
Bank Guarantee [Member]        
Line of Credit Facility, Maximum Borrowing Capacity       $ 3,000,000
Line of Credit Facility, Remaining Borrowing Capacity $ 1,000,000 1,000,000    
Letters of Credit Outstanding, Amount $ 2,000,000 $ 2,000,000    
Line of Credit Facility, Interest Rate at Period End       1.75%