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Disclosure of Compensation Related Costs, Share Based Payments (Narrative) (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
9 Months Ended
Jan. 31, 2013
Jan. 31, 2012
Compensation Related Costs, Share Based Payments [Line Items]    
Shares issued, effect on number available for grant, description Each share issued pursuant to a stock option will reduce the shares available for grant by one, and each share issued pursuant to an award of restricted stock or restricted stock units will reduce the shares available for grant by two.  
Aggregate intrinsic value of options outstanding $ 18,196  
Weighted average remaining contractual life of options outstanding (years) 6 years 8 months 12 days  
Options vested 452,811  
Weighted average exercise price of options vested $ 24.36  
Weighted average remaining contractual life of vested options (years) 5 years 1 month 6 days  
Aggregate intrinsic value of options vested 13,750  
Aggregate intrinsic value of options exercised 5,759  
Total compensation costs 3,165 2,657
2009 Stock Incentive Plan [Member]
   
Compensation Related Costs, Share Based Payments [Line Items]    
Shares available for grant 4,334,608  
Restricted Stock Units (RSUs) [Member]
   
Compensation Related Costs, Share Based Payments [Line Items]    
Shares granted 46,998  
Unrecognized compensation costs 2,028  
Stock Options [Member]
   
Compensation Related Costs, Share Based Payments [Line Items]    
Shares granted     
Unrecognized compensation costs $ 2,974  
Certain Officers And Key Employees [Member]
   
Compensation Related Costs, Share Based Payments [Line Items]    
Shares granted date 2012-06-08  
Date vested 2015-05-01  
Certain Officers And Key Employees [Member] | Restricted Stock Units (RSUs) [Member]
   
Compensation Related Costs, Share Based Payments [Line Items]    
Shares granted 32,998  
Non Employee Membes Of The Board [Member] | Restricted Stock Units (RSUs) [Member]
   
Compensation Related Costs, Share Based Payments [Line Items]    
Shares granted 14,000  
Date vested May 1, 2013