Consolidated Statements of Stockholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Common Stock in Treasury
Beginning Balance at Mar. 31, 2012	$ 243,203	$ 1,470	$ 50,383	$ 328,921	$ (25)	$ (137,546)
Beginning Balance, shares at Mar. 31, 2012		14,703,084				5,023,916
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adjustment (see Note 7)			(1,727)	1,727
Share-based compensation expense	1,783		1,783
Issuance of common stock upon exercise of stock options	192			(193)		385
Issuance of common stock upon exercise of stock options, shares						11,000
Issuance of common stock under equity plan	(262)			(1,487)		1,225
Issuance of common stock under equity plan, shares						28,784
Purchase of treasury shares	(4,864)					(4,864)
Purchase of treasury shares, shares	(251,180)					(251,180)
Tax effect of stock awards	(6)		(6)
Reduction of deferred tax assets due to expired stock options	(549)		(549)
Cash dividends	(5,731)			(5,731)
Postretirement medical plan, net of tax	(15)				(15)
Net income	15,227			15,227
Ending Balance at Mar. 31, 2013	248,978	1,470	49,884	338,464	(40)	(140,800)
Ending Balance, shares at Mar. 31, 2013		14,703,084				5,235,312
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation expense	1,843		1,843
Issuance of common stock upon exercise of stock options	49			(2,044)		2,093
Issuance of common stock upon exercise of stock options, shares						59,793
Issuance of common stock under equity plan	(563)			(2,083)		1,520
Issuance of common stock under equity plan, shares						39,928
Purchase of treasury shares	(6,634)					(6,634)
Purchase of treasury shares, shares	(272,655)					(272,655)
Tax effect of stock awards	425		425
Reduction of deferred tax assets due to expired stock options	(35)		(35)
Cash dividends	(5,637)			(5,637)
Postretirement medical plan, net of tax	21				21
Net income	18,769			18,769
Ending Balance at Mar. 31, 2014	257,216	1,470	52,117	347,469	(19)	(143,821)
Ending Balance, shares at Mar. 31, 2014		14,703,084				5,408,246
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation expense	2,038		2,038
Issuance of common stock upon exercise of stock options	46			(760)		806
Issuance of common stock upon exercise of stock options, shares	70,438					22,899
Issuance of common stock under equity plan	(293)			(1,318)		1,025
Issuance of common stock under equity plan, shares						26,013
Tax effect of stock awards	268		268
Reduction of deferred tax assets due to expired stock options	(24)		(24)
Cash dividends	(5,878)			(5,878)
Postretirement medical plan, net of tax	(72)				(72)
Net income	16,954			16,954
Ending Balance at Mar. 31, 2015	$ 270,255	$ 1,470	$ 54,399	$ 356,467	$ (91)	$ (141,990)
Ending Balance, shares at Mar. 31, 2015		14,703,084				5,359,334