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, 2020	Jan. 31, 2020	Oct. 31, 2019	Jul. 31, 2019	Apr. 30, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2020	Apr. 30, 2019	Apr. 30, 2018
Revenues	$ 195,689	$ 186,734	$ 190,310	$ 171,878	$ 176,882	$ 161,054	$ 167,171	$ 164,015	$ 744,611	$ 669,122	$ 612,201
Gross profit	69,662	65,749	67,917	61,189	63,569	58,063	61,045	59,933	264,517	242,610
Net income	9,259	12,686	13,887	15,511	14,565	10,895	11,281	10,884	51,343	47,625
Net income attributable to common stockholders	$ 9,249	$ 12,676	$ 13,877	$ 15,501	$ 14,555	$ 10,885	$ 11,271	$ 10,874	$ 51,303	$ 47,585	$ 36,469
Basic (in dollars per share)	$ 1.40	$ 1.92	$ 2.10	$ 2.32	$ 2.17	$ 1.61	$ 1.64	$ 1.57	$ 7.74	$ 6.99	$ 5.04
Diluted (in dollars per share)	$ 1.35	$ 1.83	$ 2	$ 2.21	$ 2.07	$ 1.55	$ 1.58	$ 1.53	$ 7.39	$ 6.73	$ 4.90