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Basic and Diluted Earnings per Common Share Attributable to Common Stockholders (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended 6 Months Ended
Oct. 31, 2015
Oct. 31, 2014
Oct. 31, 2015
Oct. 31, 2014
Earnings Per Share Disclosure [Line Items]        
Net income $ 17,971 $ 25,403 $ 41,053 $ 39,936
Less: distributed and undistributed earnings to nonvested restricted stockholders 167   390  
Basic net earnings attributable to common stockholders 17,804 25,403 40,663 39,936
Add: undistributed earnings to nonvested restricted stockholders 118   292  
Less: reallocation of undistributed earnings to nonvested restricted stockholders 117   289  
Diluted net earnings attributable to common stockholders $ 17,805 $ 25,403 $ 40,666 $ 39,936
Basic weighted-average number of common shares outstanding 49,981 49,082 49,737 48,893
Diluted weighted-average number of common shares outstanding 50,362 49,740 50,233 49,720
Basic earnings per share $ 0.36 $ 0.52 $ 0.82 $ 0.82
Diluted earnings per share $ 0.35 $ 0.51 $ 0.81 $ 0.80
ESPP        
Earnings Per Share Disclosure [Line Items]        
Stock 2   5  
Restricted Stock        
Earnings Per Share Disclosure [Line Items]        
Stock 330 554 433 710
Stock Options        
Earnings Per Share Disclosure [Line Items]        
Stock 49 104 58 117