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Employee Stock Plans - Additional Information (Detail) (USD $)
 3 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended
Oct. 31, 2012
Oct. 31, 2011
Oct. 31, 2012
Oct. 31, 2011
Sep. 27, 2012
Oct. 31, 2012
Common Stock
Jul. 31, 2012
Common Stock
Oct. 31, 2011
Common Stock
Jul. 31, 2011
Common Stock
Oct. 31, 2012
Common Stock
Oct. 31, 2011
Common Stock
Oct. 31, 2012
Employee Stock Purchase Plan
Oct. 31, 2011
Employee Stock Purchase Plan
Oct. 31, 2012
Time-based restricted stock
Minimum
Oct. 31, 2012
Time-based restricted stock
Maximum
Oct. 31, 2012
Market-based restricted stock
Oct. 31, 2012
Restricted Stock
Oct. 31, 2011
Restricted Stock
Oct. 31, 2012
Restricted Stock
Oct. 31, 2011
Restricted Stock
Oct. 31, 2012
Stock Options And Stock Appreciation Rights
Oct. 31, 2012
Stock Incentive Plan 2008
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                            
Weighted-average assumptions used to estimate fair value of stock-based compensation, expected volatility       47.07%                                    
Weighted-average assumptions used to estimate fair value of stock-based compensation, risk-free interest rate       1.47%                                    
Weighted-average assumptions used to estimate fair value of stock-based compensation, expected life (in years)       5 years                                    
Weighted-average assumptions used to estimate fair value of stock-based compensation, dividend yield rate       0.00%                                    
Shares available for future issuance         5,700,000             1,600,000                    
Issuance of full-value stock awards limitation, required ratio to stock option and stock appreciation rights                                           1.91
Total unrecognized compensation cost related to non-vested awards                                 $ 23,000,000   $ 23,000,000   $ 1,200,000  
Expected cost recognized over weighted-average period                                     2 years 3 months 18 days   10 months 24 days  
Vesting period                           3 years 4 years 3 years            
Shares repurchased during the period                                   9,080 6,292 186,206 182,950    
Common stock repurchased, value                                 100,000 100,000 2,600,000 4,100,000    
Authorized payroll deductions                       15.00%                    
Authorized payroll deductions, value                       25,000                    
Fair market price of common stock                       85.00%                    
Employees stock purchased                         76,909                  
Employees stock purchased, per share value                         $ 18.69                  
Shares reserved for issuance                       1,500,000                    
Amended shares reserved for issuance                       3,000,000                    
Stock issued for stock options exercised           111,230   2,500   135,511 116,186                   136,000  
Cash proceeds from exercise of stock options $ 800,000 $ 100,000 $ 1,000,000 $ 1,700,000