Net Earnings Per Share (Schedule of Earnings Per Share, Basic and Diluted) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Jan. 31, 2017	Jan. 31, 2016	Jan. 31, 2017	Jan. 31, 2016
Earnings Per Share [Abstract]
Net income	$ 14,553	$ 12,013	$ 53,851	$ 45,351
Denominator for basic net earnings per common share - weighted-average shares	16,241,670	16,294,889	16,267,333	16,252,876
Effect of dilutive securities:
Stock options and restricted stock units	140,000	162,000	134,000	198,000
Denominator for diluted net earnings per common share - weighted-average shares and assumed conversions	16,381,223	16,457,308	16,400,842	16,450,394
Net earnings per share
Basic (in usd per share)	$ 0.90	$ 0.74	$ 3.31	$ 2.79
Diluted (in usd per share)	$ 0.89	$ 0.73	$ 3.28	$ 2.76