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Note 5 - Deferred Revenue (Tables)
 12 Months Ended
Dec. 31, 2018
Notes Tables  
Deferred Revenue, by Arrangement, Disclosure [Table Text Block]
   
December 31,
  
   
201
8
    
201
7
  
Balance at beginning of year
   $
10,719
     $
9,431
  
Add: Payments received
    
14,882
      
14,369
  
Less: Revenue recognized
    
(13,746
 )    
(13,081
 )
Balance at end of year
   $
11,855
     $
10,719