Basic and Diluted Earnings per Common Share Attributable to Common Stockholders (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Jul. 31, 2015	Jul. 31, 2014
Earnings Per Share Disclosure [Line Items]
Net income	$ 23,082	$ 14,533
Less: distributed and undistributed earnings to nonvested restricted stockholders	224
Basic net earnings attributable to common stockholders	22,858	14,533
Add: undistributed earnings to nonvested restricted stockholders	176
Less: reallocation of undistributed earnings to nonvested restricted stockholders	174
Diluted net earnings attributable to common stockholders	$ 22,860	$ 14,533
Basic weighted-average number of common shares outstanding	49,493	48,703
Diluted weighted-average number of common shares outstanding	50,014	49,591
Basic earnings per share	$ 0.46	$ 0.30
Diluted earnings per share	$ 0.46	$ 0.29
ESPP
Earnings Per Share Disclosure [Line Items]
Stock	3
Restricted Stock
Earnings Per Share Disclosure [Line Items]
Stock	452	756
Stock Options
Earnings Per Share Disclosure [Line Items]
Stock	66	132