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Stock-Based Compensation - Activity (Details)
1 Months Ended 3 Months Ended 4 Months Ended 9 Months Ended
Jan. 31, 2016
item
shares
Apr. 30, 2016
USD ($)
$ / shares
shares
Apr. 30, 2015
USD ($)
Apr. 30, 2016
USD ($)
$ / shares
shares
Apr. 30, 2016
USD ($)
$ / shares
shares
Apr. 30, 2015
USD ($)
$ / shares
Jul. 31, 2015
$ / shares
shares
Stock-Based Compensation              
Total unrecognized stock-based compensation expense, before income taxes, related to total nonvested stock options and stock awards (in dollars) | $   $ 11,005,000   $ 11,005,000 $ 11,005,000    
Remaining weighted average period over which unrecognized stock-based compensation expense is expected to be recognized         18 months    
Number of Shares              
Outstanding at the beginning of the period (in shares)         107,500    
Granted (in shares)         15,000    
Outstanding at the end of the period (in shares)   122,500   122,500 122,500    
Exercisable at the end of the period (in shares)   80,834   80,834 80,834   45,000
Weighted Average Exercise Price              
Outstanding at the beginning of the period (in dollars per share) | $ / shares         $ 25.73    
Granted (in dollars per share) | $ / shares         55.36    
Outstanding at the end of the period (in dollars per share) | $ / shares   $ 29.36   $ 29.36 29.36    
Exercisable at the end of the period (in dollars per share) | $ / shares   $ 22.72   $ 22.72 $ 22.72   $ 20.32
Stock-based awards, additional disclosure              
Deduction in income tax due to exercise of options and vesting of restricted stock (in dollars) | $         $ 3,297,000 $ 4,532,000  
Increase in additional paid-in capital due to excess tax benefit on stock-based compensation expense (in dollars) | $         $ 1,586,000 $ 2,662,000  
2006 Plan              
Stock-based awards, additional disclosure              
Shares available for grant 0            
2016 Plan              
Stock-based awards, additional disclosure              
Maximum number of shares authorized for issuance 1,200,000            
Restricted shares              
Number of Shares              
Nonvested stock awards at the beginning of the period (in shares)         343,519    
Granted (in shares)         167,990    
Cancelled (in shares)         (4,324)    
Vested (in shares)         (169,575)    
Nonvested stock awards at the end of the period (in shares)   337,610   337,610 337,610    
Weighted Average Fair Value              
Nonvested stock awards at the beginning of the period (in dollars per share) | $ / shares         $ 32.77    
Granted (in dollars per share) | $ / shares         54.81    
Cancelled (in dollars per share) | $ / shares         44.04    
Vested (in dollars per share) | $ / shares         28.49    
Nonvested stock awards at the end of the period (in dollars per share) | $ / shares   $ 45.74   $ 45.74 $ 45.74    
Restricted shares | 2016 Plan              
Number of Shares              
Granted (in shares)       5,725      
Stock-based awards, additional disclosure              
Number of anniversaries of grant date on which awards vest | item 3            
Restricted shares | 2016 Plan | First anniversary vesting              
Stock-based awards, additional disclosure              
Vesting percentage 33.30%            
Vesting period 1 year            
Restricted shares | 2016 Plan | Second anniversary vesting              
Stock-based awards, additional disclosure              
Vesting percentage 33.30%            
Vesting period 2 years            
Restricted shares | 2016 Plan | Third anniversary vesting              
Stock-based awards, additional disclosure              
Vesting percentage 33.30%            
Vesting period 3 years            
Stock options              
Weighted-Average Black-Scholes Option Valuation Assumptions              
Dividend yield (as a percent)         0.22% 0.25%  
Expected volatility (as a percent)         55.90% 33.90%  
Risk-free interest rate (as a percent)         1.41% 1.55%  
Expected lives         5 years 5 years  
Stock options, additional disclosure              
Weighted average fair value of all options granted (in dollars per share) | $ / shares         $ 26.49 $ 11.54  
Aggregate intrinsic value of all options exercised (in dollars) | $   $ 0 $ 383,000   $ 0 $ 2,803,000