Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Oct. 31, 2017	Oct. 31, 2016	Oct. 31, 2017	Oct. 31, 2016
Computation of basic and diluted earnings per share of common stock
Net income	$ 18,023	$ 17,224	$ 33,366	$ 26,387
Basic earnings per common share:
Basic weighted average common shares outstanding (in shares)	41,006	40,943	40,988	39,579
Basic earnings per common share (in dollars per share)	$ 0.44	$ 0.42	$ 0.81	$ 0.67
Diluted earnings per common share:
Basic weighted average common shares outstanding (in shares)	41,006	40,943	40,988	39,579
Add: Shares of common stock assumed issued upon exercise of stock options (in shares)	1,140	377	1,149	377
Diluted weighted average common shares outstanding (in shares)	42,146	41,320	42,137	39,956
Diluted earnings per common share (in dollars per share)	$ 0.43	$ 0.42	$ 0.79	$ 0.66