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Stock-based Compensation (Details) (USD $)
 12 Months Ended 0 Months Ended 0 Months Ended 0 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Mar. 31, 2011
Mar. 31, 2013
Stock options
Mar. 31, 2012
Stock options
Mar. 31, 2011
Stock options
Mar. 31, 2010
Stock options
Mar. 31, 2013
Stock options
Minimum
Mar. 31, 2013
Stock options
Maximum
Feb. 27, 2013
2006 new stock compensation plan
Jun. 22, 2010
2006 new stock compensation plan
Dec. 08, 2006
2006 new stock compensation plan
Mar. 31, 2013
2006 new stock compensation plan
Stock options
Oct. 18, 2004
1999 stock compensation plan
Mar. 31, 2013
1999 stock compensation plan
Oct. 21, 1999
1999 stock compensation plan
Mar. 31, 2013
1999 stock compensation plan
Stock options
Mar. 31, 2013
Nonqualified performance stock option plan
Oct. 03, 1996
Nonqualified performance stock option plan
Maximum
Stock based compensation                                      
Price of option granted as a percentage of quoted price of the underlying shares       100.00%                              
Number of shares reserved for issuance                     800,000 400,000       300,000      
Number of options outstanding       416,125 433,785 443,642 391,765           382,750       33,375    
Additional number of shares reserved for issuance                   400,000       200,000          
Number of shares that may be granted per director per year                                     20,000
Amounts recognized in the condensed financial statements related to stock-based compensation                                      
Total cost of stock-based compensation charged against income before income tax $ 1,112,000 $ 464,000 $ 383,000                                
Amount of income tax benefit recognized in earnings 77,000 81,000 21,000                                
Amount charged against net income 1,035,000 383,000 362,000                                
Impact on net income per common share:                                      
Basic (in dollars per share) $ 0.31 $ 0.12 $ 0.11                                
Diluted (in dollars per share) $ 0.29 $ 0.11 $ 0.11                                
Assumptions used to estimate the fair value of options granted                                      
Volatility, minimum (as a percent)       27.50% 33.40% 34.00%                          
Volatility, maximum (as a percent)       31.10% 33.70% 36.00%                          
Risk-free interest rate, minimum (as a percent)       0.60% 0.90% 1.10%                          
Risk-free interest rate, maximum (as a percent)       1.00% 2.20% 3.90%                          
Expected option life               5 years 10 years                    
Dividend yield (as a percent)       1.40% 1.80% 1.80%                          
Number of Shares                                      
Outstanding at the beginning of the period (in shares)       433,785 443,642 391,765             382,750       33,375    
Granted (in shares)       116,080 103,780 137,060                          
Forfeited (in shares)       (40,375) (11,940) (22,315)                          
Expired (in shares)       (40) (1,020) (150)                          
Exercised (in shares)       (93,325) (100,677) (62,718)                          
Outstanding at the end of the period (in shares)       416,125 433,785 443,642 391,765           382,750       33,375    
Exercisable at the end of the period (in shares)       158,320 148,910 152,217                          
Weighted-average Exercise Price per Share                                      
Outstanding at the beginning of the period (in dollars per share)       $ 22.77 $ 20.10 $ 17.37                          
Granted (in dollars per share)       $ 49.97 $ 29.87 $ 25.43                          
Forfeited (in dollars per share)       $ 32.87 $ 26.06 $ 19.16                          
Expired (in dollars per share)       $ 18.98 $ 14.50 $ 11.65                          
Exercised (in dollars per share)       $ 20.56 $ 18.00 $ 15.02                          
Outstanding at the end of the period (in dollars per share)       $ 29.87 $ 22.77 $ 20.10 $ 17.37                        
Exercisable at the end of the period (in dollars per share)       $ 21.00 $ 19.28 $ 17.36                          
Weighted- average Remaining Contractual Term                                      
Outstanding       3 years 8 months 12 days 3 years 10 months 24 days 4 years 4 years 2 months 12 days                        
Granted       5 years 10 months 24 days 5 years 4 months 24 days 5 years                          
Exercisable at the end of the period       3 years 3 years 2 months 12 days 3 years 2 months 12 days                          
Aggregate Intrinsic Value                                      
Outstanding at the end of the period       9,529,000 11,516,000 3,861,000                          
Exercisable at the end of the period       5,031,000 4,473,000 1,742,000                          
Unvested Shares                                      
Unvested at the beginning of the period (in shares)       284,875 291,425 247,085                          
Options granted (in shares)       116,080 103,780 137,060                          
Options forfeited (in shares)       (38,720) (11,395) (13,540)                          
Options vested (in shares)       (104,415) (98,935) (79,180)                          
Unvested at the end of the period (in shares)       257,805 284,875 291,425 247,085                        
Weighted-average Grant-Date Fair Value                                      
Unvested at the beginning of the period (in dollars per share)       $ 7.28 $ 6.46 $ 5.51                          
Options granted (in dollars per share)       $ 12.43 $ 8.33 $ 7.53                          
Options forfeited (in dollars per share)       $ 8.86 $ 7.31 $ 6.51                          
Options vested (in dollars per share)       $ 6.69 $ 5.97 $ 5.34                          
Unvested at the end of the period (in dollars per share)       $ 9.55 $ 7.28 $ 6.46 $ 5.51                        
Other stock option disclosures                                      
Intrinsic value of options exercised       2,742,000 2,228,000 688,000                          
Unrecognized compensation cost       1,889,000                              
Number of shares available for future option grants       310,820                     0     0  
Number of unvested options modified (in shares)       14,400                              
Expiration term       10 years                              
Incremental compensation expense       $ 240,000