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Employee Stock Plans - Additional Information (Detail) (USD $)
3 Months Ended 6 Months Ended
Oct. 31, 2014
Oct. 31, 2013
Oct. 31, 2014
Oct. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Common stock repurchased, value     $ 3,748,000 $ 1,967,000
Cash proceeds from exercise of stock options 400,000 2,000,000 1,884,000 3,854,000
Shares repurchased during the period 0 0 0 0
Time Based Restricted Stock | Minimum
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period     3 years  
Time Based Restricted Stock | Maximum
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Vesting period     4 years  
Market Based Restricted Stock
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total unrecognized compensation cost related to non-vested awards 5,000,000   5,000,000  
Vesting period     3 years  
Shares outstanding 300,000   300,000  
Performance Based Restricted Stock Unit
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total unrecognized compensation cost related to non-vested awards 3,600,000   3,600,000  
Vesting period     3 years  
Shares outstanding 200,000   200,000  
Stock Options
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total unrecognized compensation cost related to non-vested awards 100,000   100,000  
Stock Options | Common Stock
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock issued for stock options exercised 24,308 126,244 109,629 315,123
Restricted Stock
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total unrecognized compensation cost related to non-vested awards 28,400,000   28,400,000  
Expected cost recognized over weighted-average period     2 years 6 months  
Shares repurchased during the period 662 1,025 126,083 101,399
Common stock repurchased, value $ 100,000 $ 100,000 $ 3,748,000 $ 1,967,000
Stock Incentive Plan 2008
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock incentive plan, maximum number of shares issuable 5,700,000   5,700,000  
Issuance of full-value stock awards limitation, required ratio to stock options 1.91   1.91