CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ 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
Net sales
Total	$ 325,434		$ 345,664		$ 345,043		$ 356,532		$ 329,857		$ 344,226		$ 371,363		$ 373,356		$ 1,372,673		$ 1,418,802		$ 1,331,778
Cost of sales
Total	244,162		262,751		259,543		275,829		248,227		267,538		283,792		295,763		1,042,285		1,095,320		1,032,019
Gross profit	81,272		82,913		85,500		80,703		81,630		76,688		87,571		77,593		330,388		323,482		299,759
Selling, general, and administrative	62,683		59,728		57,705		56,966		59,989		57,134		56,340		54,664		237,082		228,127		205,232
Depreciation and amortization	3,574		2,719		2,741		2,790		2,835		2,894		2,129		2,063		11,824		9,921		7,252
Interest and financing costs	545		443		484		476		292		270		274		359		1,948		1,195		1,543
Operating expenses	66,802		62,890		60,930		60,232		63,116		60,298		58,743		57,086		250,854		239,243		214,027
Operating income	14,470		20,023		24,570		20,471		18,514		16,390		28,828		20,507		79,534		84,239		85,732
Other income (expense)	5,556		721		322		97		(347)		(131)		(141)		271		6,696		(348)		380
Earnings before tax	20,026		20,744		24,892		20,568		18,167		16,259		28,687		20,778		86,230		83,891		86,112
Provision for income taxes	4,974		5,880		6,889		5,295		9,270		678		11,466		7,355		23,038		28,769		35,556
Net earnings	$ 15,052		$ 14,864		$ 18,003		$ 15,273		$ 8,897		$ 15,581		$ 17,221		$ 13,423		$ 63,192		$ 55,122		$ 50,556
Net earnings per common share-basic (in dollars per share)	$ 1.12	[1]	$ 1.10	[1]	$ 1.33	[1]	$ 1.14	[1]	$ 0.65	[1]	$ 1.12	[1]	$ 1.24	[1]	$ 0.97	[1]	$ 4.70	[1]	$ 4.00	[1]	$ 3.65
Net earnings per common share-diluted (in dollars per share)	$ 1.12	[1]	$ 1.10	[1]	$ 1.33	[1]	$ 1.12	[1]	$ 0.65	[1]	$ 1.11	[1]	$ 1.23	[1]	$ 0.96	[1]	$ 4.65	[1]	$ 3.95	[1]	$ 3.60
Weighted average common shares outstanding-basic (in shares)																	13,448		13,790		13,867
Weighted average common shares outstanding-diluted (in shares)																	13,578		13,967		14,028
Product [Member]
Net sales
Total	$ 280,460		$ 310,443		$ 309,475		$ 322,817		$ 293,232		$ 310,120		$ 339,710		$ 346,245		$ 1,223,195		$ 1,289,307		$ 1,234,344
Cost of sales
Total	216,662		241,856		238,134		255,812		228,212		247,891		266,732		280,755		952,464		1,023,590		978,479
Services [Member]
Net sales
Total	44,974		35,221		35,568		33,715		36,625		34,106		31,653		27,111		149,478		129,495		97,434
Cost of sales
Total	$ 27,500		$ 20,895		$ 21,409		$ 20,017		$ 20,015		$ 19,647		$ 17,060		$ 15,008		$ 89,821		$ 71,730		$ 53,540

[1] Basic and diluted earnings per share are computed independently for each of the quarters presented. Therefore, the sum of quarterly basic and diluted per share information may not equal annual basic and diluted earnings per share.