Note 6 - Stock-based Compensation (Tables)
 12 Months Ended
Mar. 31, 2018
Notes Tables  
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]
   
Year Ended March 31
  
   
2018
    
2017
    
2016
  
Risk-free interest rate
    
1.8
%
    
1.0
%
    
1.6
%
Expected volatility
    
28
%
    
27
%
    
26
%
Expected life (years)
    
4.4
      
4.3
      
4.3
  
Dividend yield
    
5.3
%
    
7.0
%
    
7.1
%
Schedule of Share-based Compensation, Shares Authorized under Stock Option Plans, by Exercise Price Range [Table Text Block]
Ranges of
Exercise Prices
    
Number
Outstanding
    
Weighted Average
Exercise Price
    
Weighted Remaining
Contractual Life (years)
  
$42.45
-
54.11
      
4,000
     $
49.37
      
3.4
  
 56.76
-
76.13
      
18,000
      
63.81
      
7.4
  
 
 
 
      
22,000
     $
61.19
      
6.6
  
Share-based Compensation, Activity [Table Text Block]
   
Option Shares
Reserved
    
Options
Outstanding
    
Weighted Average
Option Exercise Price
  
At March 31, 2015
    
151,230
      
35,090
     $
33.51
  
Granted
    
(4,000
)
    
4,000
     $
56.76
  
Exercised
    
-
      
(18,090
)
   $
16.19
  
At March 31, 2016
    
147,230
      
21,000
     $
52.85
  
Granted
    
(4,000
)
    
4,000
     $
57.46
  
Exercised
    
-
      
(6,000
)
   $
41.19
  
At March 31, 2017
    
143,230
      
19,000
     $
57.51
  
Granted
    
(4,000
)
    
4,000
     $
76.13
  
Exercised
    
-
      
(1,000
)
   $
51.04
  
At March 31, 2018
    
139,230
      
22,000
     $
61.19