Note N - Quarterly Results of Operations (Unaudited) - Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Apr. 30, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2018	Jan. 31, 2018	Oct. 31, 2017	Jul. 31, 2017	Apr. 30, 2019	Apr. 30, 2018	Apr. 30, 2017
Revenues	$ 176,882	$ 161,054	$ 167,171	$ 164,015	$ 169,451	$ 147,214	$ 149,118	$ 146,418	$ 669,122	$ 612,201	$ 587,751
Gross profit	63,569	58,063	61,045	59,933	61,168	53,215	54,804	53,068	242,610	222,255
Net income	14,565	10,895	11,281	10,884	10,169	13,379	5,969	6,992	47,625	36,509
Net income attributable to common stockholders	$ 14,555	$ 10,885	$ 11,271	$ 10,874	$ 10,159	$ 13,369	$ 5,959	$ 6,982	$ 47,585	$ 36,469	$ 20,165
Basic (in dollars per share)	$ 2.17	$ 1.61	$ 1.64	$ 1.57	$ 1.47	$ 1.88	$ 0.82	$ 0.92	$ 6.99	$ 5.04	$ 2.57
Diluted (in dollars per share)	$ 2.07	$ 1.55	$ 1.58	$ 1.53	$ 1.43	$ 1.82	$ 0.79	$ 0.90	$ 6.73	$ 4.90	$ 2.49