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, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Sales	$ 67,322	$ 60,110	$ 66,711	$ 79,657	$ 63,032	$ 52,866	$ 60,791	$ 72,487	$ 273,800	$ 249,176	$ 234,684
Gross Profit	24,919	21,944	23,479	27,465	22,155	16,643	18,064	22,452	97,807	79,314	76,296
Income from operations	14,004	11,468	12,248	14,416	11,697	7,655	7,731	10,400	52,136	37,483	32,388
Net income	$ 10,183	$ 9,064	$ 8,760	$ 9,276	$ 7,503	$ 4,823	$ 4,899	$ 6,594	$ 37,283	$ 23,819	$ 20,567
Diluted net income per common share (in dollars per share)	$ 0.50	$ 0.44	$ 0.43	$ 0.45	$ 0.37	$ 0.24	$ 0.24	$ 0.32	$ 1.82	$ 1.17	$ 1.02