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Stock-Based Compensation (Details) (USD $)
 3 Months Ended 6 Months Ended 12 Months Ended 6 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Stock options
Sep. 30, 2011
Stock options
Mar. 31, 2012
Stock options
Sep. 30, 2012
Stock options
Minimum
Sep. 30, 2011
Stock options
Minimum
Sep. 30, 2012
Stock options
Maximum
Sep. 30, 2011
Stock options
Maximum
Amounts recognized in the condensed financial statements related to stock-based compensation                      
Total cost of stock-based compensation charged against income before income taxes $ 289,000 $ 96,000 $ 438,000 $ 192,000              
Amount of income tax benefit recognized in earnings 101,000 35,000 151,000 70,000              
Amount charged against net income 188,000 61,000 287,000 122,000              
Impact on net income per common share:                      
Basic (in dollars per share) $ 0.06 $ 0.02 $ 0.09 $ 0.04              
Diluted (in dollars per share) $ 0.05 $ 0.02 $ 0.08 $ 0.04              
Assumptions used to estimate the fair value of options granted                      
Volatility (as a percent)         31.10% 33.40%          
Risk-free interest rate (as a percent)               0.60% 2.20% 1.00% 3.60%
Expected option life               5 years 5 years 10 years 10 years
Dividend yield (as a percent)         1.50% 1.70%          
Number of Shares                      
Outstanding at the beginning of the period (in shares)         433,785            
Options granted (in shares)         103,880            
Options forfeited (in shares)         (31,050)            
Options expired (in shares)         (40)            
Options exercised (in shares)         (47,495)            
Outstanding at the end of the period (in shares)         459,080   433,785        
Exercisable at the end of the period (in shares)         192,530            
Weighted-average Exercise Price per Share                      
Outstanding at the beginning of the period (in dollars per share)         $ 22.77            
Options granted (in dollars per share)         $ 50.20            
Options forfeited (in dollars per share)         $ 33.06            
Options expired (in dollars per share)         $ 18.98            
Options exercised (in dollars per share)         $ 21.56            
Outstanding at the end of the period (in dollars per share)         $ 28.41   $ 22.77        
Exercisable at the end of the period (in dollars per share)         $ 20.70            
Weighted- average Remaining Contractual Term                      
Outstanding             3 years 10 months 24 days        
Options granted         5 years 10 months 24 days            
Exercisable at the end of the period         3 years 1 month 6 days            
Aggregate Intrinsic Value                      
Outstanding at the end of the period         13,041,000            
Exercisable at the end of the period         3,986,000            
Weighted average grant date fair value of option granted and intrinsic value of option exercised                      
Intrinsic value of options exercised         1,300,000 588,000          
Number of Shares                      
Unvested at the beginning of the period (in shares)         284,875            
Options granted (in shares)         103,880            
Options forfeited (in shares)         (31,050)            
Options vested (in shares)         (91,155)            
Unvested at the end of the period (in shares)         266,550   284,875        
Weighted-average Grant-Date Fair Value                      
Unvested at the beginning of the period (in dollars per share)         $ 7.23            
Options granted (in dollars per share)         $ 12.37            
Options forfeited (in dollars per share)         $ 8.92            
Options vested (in dollars per share)         $ 6.80            
Unvested at the end of the period (in dollars per share)         $ 9.18   $ 7.23        
Unrecognized compensation cost                      
Unrecognized compensation cost         $ 1,930,000            
Weighted average period of recognition         3 years