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Stock-Based Compensation (Details 2) (USD $)
3 Months Ended 6 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended
Jul. 31, 2012
Jan. 31, 2013
Apr. 30, 2013
Apr. 30, 2012
Apr. 30, 2013
Restricted shares
Apr. 30, 2013
Stock options
Apr. 30, 2012
Stock options
Apr. 30, 2013
Stock options
Apr. 30, 2012
Stock options
Jul. 31, 2012
Stock options
Stock-Based Compensation                    
Total unrecognized stock-based compensation expense, before income taxes, related to total nonvested stock options and stock awards     $ 5,371,000              
Remaining weighted average period over which unrecognized stock-based compensation expense is expected to be recognized     19 months              
Number of Shares                    
Nonvested stock awards at the beginning of the period (in shares)         472,005          
Granted (in shares)         132,692          
Cancelled (in shares)         (8,830)          
Vested (in shares)         (181,435)          
Nonvested stock awards at the end of the period (in shares)         414,432          
Weighted Average Fair Value                    
Nonvested stock awards at the beginning of the period (in dollars per share)         $ 13.73          
Granted (in dollars per share)         $ 25.60          
Cancelled (in dollars per share)         $ 17.23          
Vested (in dollars per share)         $ 13.06          
Nonvested stock awards at the end of the period (in dollars per share)         $ 17.75          
Weighted-Average Black-Scholes Option Valuation Assumptions                    
Dividend yield (as a percent)               0.37%    
Expected volatility (as a percent)               0.509%    
Risk-free interest rate (as a percent)               0.67%    
Expected lives               5 years    
Stock options, additional disclosure                    
Weighted average fair value of all options granted (in dollars per share)     $ 10.91              
Aggregate intrinsic value of all options exercised (in dollars)           537,000 1,566,000 4,025,000 3,413,000  
Number of Shares                    
Outstanding at the beginning of the period (in shares)               548,823    
Options granted (in shares)             0 35,000 0  
Canceled (in shares)               (6,000)    
Exercised (in shares)               (209,266)    
Outstanding at the end of the period (in shares)           368,557   368,557    
Exercisable at the end of the period (in shares)           333,557   333,557   364,110
Weighted Average Exercise Price                    
Outstanding at the beginning of the period (in dollars per share)               $ 9.86    
Granted (in dollars per share)               $ 25.56    
Canceled (in dollars per share)               $ 12.61    
Exercised (in dollars per share)               $ 9.80    
Outstanding at the end of the period (in dollars per share)           $ 11.34   $ 11.34    
Exercisable at the end of the period (in dollars per share)           $ 9.85   $ 9.85   $ 9.43
Options vested or expected to vest                    
Reissued shares from treasury stock for exercise of stock options and grant of stock awards (in shares) 107,269 316,177                
Reduction in income tax payable due to exercise of options and vesting of restricted stock     3,073,000 2,435,000            
Increase in additional paid-in capital due to excess tax benefit on stock-based compensation expense     $ 1,986,000 $ 1,315,000