Note 5 - Marketable Securities (Tables)
 6 Months Ended
Sep. 30, 2017
Notes Tables  
Marketable Securities [Table Text Block]
Total
    
<1 Year
    
1
–3 Years
    
3
–5 Years
  
$ 76,544,498     $
16,921,738
     $
24,122,782
     $
35,499,978
  
Schedule of Available-for-sale Securities Reconciliation [Table Text Block]
   
As of September
30, 2017
    
As of March
31, 2017
  
   

Adjusted
Cost
    
Gross
Unrealized
Gains
    
Gross
Unrealized
Losses
    
Fair
Market
Value
    

Adjusted
Cost
    
Gross
Unrealized
Gains
    
Gross
Unrealized
Losses
    
Fair
Market
Value
  
Corporate
bonds
   $
76,498,214
     $
197,573
     $
(151,289
)
   $
76,544,498
     $
75,158,087
     $
187,001
     $
(246,935
)
   $
75,098,153
  
Municipal
bonds
   
-
      
-
      
-
      
-
      
1,304,817
      
-
      
(214
)
    
1,304,603
  
Total
   $
76,498,214
     $
197,573
     $
(151,289
)
   $
76,544,498
     $
76,462,904
     $
187,001
     $
(247,149
)
   $
76,402,756
  
Schedule of Unrealized Loss on Investments [Table Text Block]
   
Less Than 12 Months
    
12 Months or Greater
    
Total
  
   
Fair
Market
Value
    
Gross
Unrealized
Losses
    
Fair
Market
Value
    
Gross
Unrealized
Losses
    
Fair
Market
Value
    
Gross
Unrealized
Losses
  
As of September
30, 2017
  
Corporate bonds
   $
22,385,041
     $
(113,584
)
   $
7,611,641
     $
(37,705
)
   $
29,996,682
     $
(151,289
)
Municipal
bonds
   
-
      
-
      
-
    
 
-
      
-
      
-
  
Total
   $
22,385,041
     $
(113,584
)
   $
7,611,641
     $
(37,705
)
   $
29,996,682
     $
(151,289
)
As of March 31, 2017
  
Corporate bonds
   $
32,198,766
     $
(246,935
)
   $
-
     $
-
     $
32,198,766
     $
(246,935
)
Municipal
bonds
   
1,304,603
      
(214
)
    
-
      
-
      
1,304,603
      
(214
)
Total
   $
33,503,369
     $
(247,149
)
   $
-
     $
-
     $
33,503,369
     $
(247,149
)