Net Earnings Per Share (Schedule of Earnings Per Share, Basic and Diluted) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Oct. 31, 2017	Oct. 31, 2016	Oct. 31, 2017	Oct. 31, 2016
Earnings Per Share [Abstract]
Net income	$ 19,755	$ 17,637	$ 42,036	$ 39,298
Denominator for basic net earnings per common share - weighted-average shares	16,197,088	16,295,948	16,234,746	16,280,164
Effect of dilutive securities:
Stock options and restricted stock units	71,000	144,000	84,000	131,000
Denominator for diluted net earnings per common share - weighted-average shares and assumed conversions	16,268,078	16,440,321	16,319,224	16,410,652
Net earnings per share
Basic (in usd per share)	$ 1.22	$ 1.08	$ 2.59	$ 2.41
Diluted (in usd per share)	$ 1.21	$ 1.07	$ 2.58	$ 2.39