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Note 4 - Marketable Securities (Tables)
 6 Months Ended
Jun. 30, 2018
Notes Tables  
Schedule of Available-for-sale Securities Reconciliation [Table Text Block]
   
Amortized cost
    
Gross unrealized
holding gains
    
Gross unrealized
holding losses
    
Estimated
fair value
  
June 30
, 2018
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
 
Available-for-sale securities:
                                
Corporate bonds and notes
   $
4,857
     $
—
     $
(62
)
   $
4,795
  
Municipal bonds
    
4,491
      
—
      
(43
)
    
4,448
  
Total available-for-sale securities
   $
9,348
     $
—
     $
(105
)
   $
9,243
  
   
Amortized
cost
    
Gross unrealized
holding gains
    
Gross unrealized
holding losses
    
Estimated
fair value
  
December 31, 2017
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
 
Available-for-sale securities:
                                
Corporate bonds and notes
   $
8,458
     $
19
     $
(49
)
   $
8,428
  
Municipal bonds
    
4,637
      
1
      
(28
)
    
4,610
  
Total available-for-sale securities
   $
13,095
     $
20
     $
(77
)
   $
13,038
  
Available-for-sale Securities, Debt Maturities [Table Text Block]
   
Amortized
cost
    
Estimated
fair value
  
June 30, 2018
  
 
 
 
  
 
 
 
Due within one year
   $
2,862
     $
2,852
  
Due after one year through five years
    
6,332
      
6,239
  
Due after five years
    
154
      
152
  
Total available-for-sale securities
   $
9,348
     $
9,243
  
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Table Text Block]
   
Less than 12 months
    
More than 12 months
    
Total
  
(In thousands)
  
Estimated
fair value
    
Gross unrealized
holding losses
    
Estimated
fair value
    
Gross unrealized
holding losses
    
Estimated
fair value
    
Gross unrealized
holding losses
  
As of
June 30
, 2018
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
 
Corporate bonds and notes
   $
2,672
     $
(24
)
   $
1,866
     $
(38
)
   $
4,538
     $
(62
)
Municipal bonds
    
3,325
      
(32
)
    
919
      
(11
)
    
4,244
      
(43
)
Total
   $
5,997
     $
(56
)
   $
2,785
     $
(49
)
   $
8,782
     $
(105
)
   
Less than 12 months
    
More than 12 months
    
Total
  
(In thousands)
  
Estimated
fair value
    
Gross unrealized
holding losses
    
Estimated
fair value
    
Gross unrealized
holding losses
    
Estimated
fair value
    
Gross unrealized
holding losses
  
As of December 31, 2017
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
 
  
 
 
 
Corporate bonds and notes
   $
3,799
     $
(20
)
   $
2,125
     $
(30
)
   $
5,924
     $
(50
)
Municipal bonds
    
3,341
      
(18
)
    
657
      
(9
)
    
3,998
      
(27
)
Total
   $
7,140
     $
(38
)
   $
2,782
     $
(39
)
   $
9,922
     $
(77
)