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Note 4 - Financing Arrangements (Details Textual) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Jun. 30, 2016
Interest Expense, Debt $ 17,000 $ 20,000 $ 122,000  
Operating Leases, Rent Expense 900,000 1,100,000 $ 1,000,000  
Line of Credit Maturing February 2020 [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 $ 18,300,000 20,000,000    
Letters of Credit Outstanding, Amount $ 1,700,000      
Line of Credit Maturing February 2020 [Member] | London Interbank Offered Rate (LIBOR) [Member]        
Debt Instrument, Basis Spread on Variable Rate 0.75%      
Line of Credit Maturing May 2018 [Member]        
Line of Credit Facility, Maximum Borrowing Capacity $ 8,000,000      
Line of Credit Facility, Remaining Borrowing Capacity 2,700,000 3,200,000    
Letters of Credit Outstanding, Amount $ 5,300,000 4,800,000    
Line of Credit Maturing May 2018 [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 $ 800,000 600,000    
Letters of Credit Outstanding, Amount $ 2,200,000 $ 2,400,000    
Line of Credit Facility, Interest Rate at Period End       1.75%