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Note 8 - Contract Balance (Tables)
 9 Months Ended
Sep. 30, 2018
Notes Tables  
Deferred Revenue, by Arrangement, Disclosure [Table Text Block]
   
Three Months Ended
    
Nine Months Ended
  
   
September 30,
    
September 30,
  
   
2018
    
2017
    
2018
    
2017
  
Beginning Balance
   $
11,807
     $
10,013
     $
11,656
     $
9,431
  
Add: Payments received
    
3,609
      
3,178
      
12,025
      
10,290
  
Less: Revenue recognized
    
(4,097)
      
(3,233)
 
    
(12,362)
      
(9,763)
  
Ending Balance
   $
11,319
     $
9,958
     $
11,319
     $
9,958