Note 13 - Quarterly Financial Data (Unaudited) - Summarized Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Sales	$ 64,565	$ 60,068	$ 71,396	$ 87,390	$ 67,322	$ 60,110	$ 66,711	$ 79,657
Gross Profit	20,724	19,381	25,255	29,954	24,919	21,944	23,479	27,465	$ 95,314	$ 97,807	$ 79,314
Income from operations	7,802	9,413	13,203	15,757	14,004	11,468	12,248	14,416	46,174	52,136	37,483
Net income	$ 6,620	$ 7,787	$ 10,752	$ 12,582	$ 10,183	$ 9,064	$ 8,760	$ 9,276	$ 37,740	$ 37,283	$ 23,819
Diluted net income per common share (in dollars per share)	$ 0.32	$ 0.38	$ 0.52	$ 0.62	$ 0.50	$ 0.44	$ 0.43	$ 0.45	$ 1.84	$ 1.82	$ 1.17