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Employee Stock Plans - Additional Information (Detail) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended 9 Months Ended
Jan. 31, 2014
Jan. 31, 2013
Jan. 31, 2014
Jan. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock options granted 0 0 0 0
Shares repurchased during the period 0 0 0 0
Cash proceeds from exercise of stock options $ 3.6 $ 0.3 $ 7.4 $ 1.3
Common Stock
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock issued for stock options exercised 255,732 35,083 570,855 170,594
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  
Stock Options
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total unrecognized compensation cost related to non-vested awards 0.2   0.2  
Expected cost recognized over weighted-average period     7 months 6 days  
Stock issued for stock options exercised     571,000  
Market Based Restricted Stock
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total unrecognized compensation cost related to non-vested awards 2.6   2.6  
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 4.8   4.8  
Vesting period     3 years  
Shares outstanding 200,000   200,000  
Restricted Stock
       
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Total unrecognized compensation cost related to non-vested awards 25.4   25.4  
Expected cost recognized over weighted-average period     2 years 6 months  
Shares repurchased during the period 2,308 2,065 103,707 188,271
Common stock repurchased, value $ 0.1 $ 0.1 $ 2.1 $ 2.7
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