Reconciliation of Net Income Amounts and Weighted Average Number of Common and Common Equivalent Shares Used to Determine Basic and Diluted Earnings per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 31, 2016	Jan. 31, 2015	Jan. 31, 2016	Jan. 31, 2015
Earnings Per Share [Abstract]
Net income	$ 31,434	$ 8,121	$ 58,311	$ 27,725
Basic earnings, Shares	54,857	53,724	54,508	54,033
Effect of dilutive stock awards, Shares	1,124	1,135	1,276	1,225
Diluted earnings, Shares	55,981	54,859	55,784	55,258
Basic	$ 0.57	$ 0.15	$ 1.07	$ 0.51
Effect of dilutive stock awards	(0.01)		(0.02)	(0.01)
Diluted earnings	$ 0.56	$ 0.15	$ 1.05	$ 0.50