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, 2018	Jan. 31, 2017	Jan. 31, 2018	Jan. 31, 2017
Earnings Per Share [Abstract]
Net income	$ 1,996	$ 14,553	$ 44,032	$ 53,851
Denominator for basic net earnings per common share - weighted-average shares	16,578,235	16,241,670	16,349,716	16,267,333
Effect of dilutive securities:
Stock options and restricted stock units	113,000	140,000	112,000	134,000
Denominator for diluted net earnings per common share - weighted-average shares and assumed conversions	16,690,760	16,381,223	16,461,509	16,400,842
Net earnings per share
Basic (in usd per share)	$ 0.12	$ 0.90	$ 2.69	$ 3.31
Diluted (in usd per share)	$ 0.12	$ 0.89	$ 2.67	$ 3.28