Selected Balance Sheet Information (Details) (USD $) In Thousands, unless otherwise specified	Jul. 31, 2014	Apr. 30, 2014	Jan. 31, 2014	Jul. 31, 2013	Apr. 30, 2013	Jan. 31, 2013
Accounts receivable, net:
Trade accounts	$ 52,797		$ 54,962	$ 49,404
Allowance for doubtful accounts	(344)		(319)	(205)
Accounts receivable, net	52,453		54,643	49,199
Inventories:
Finished goods	7,098		7,232	7,980
In process	1,694		2,131	2,562
Materials	42,464		45,502	43,820
Inventories	51,256		54,865	54,362
Property, plant and equipment, net:
Property, plant and equipment	189,349		182,700	171,392
Accumulated depreciation	(91,543)		(84,624)	(81,127)
Property, plant and equipment, net	97,806		98,076	90,265
Accrued liabilities:
Salaries and benefits	2,211		1,858	2,391
Vacation	3,563		3,700	4,171
401(k) contributions	309		727	417
Insurance obligations	2,166		2,428	2,321
Warranties	2,617	2,654	2,525	2,047	1,990	1,888
Acquisition-related contingent liabilities	919		890	594
Taxes - accrued and withheld	1,387		1,743	1,900
Other	2,104		2,377	1,450
Accrued liabilities	15,276		16,248	15,291
Other liabilities:
Postretirement benefits	8,166		7,998	8,208
Acquisition-related contingent consideration,	3,682		2,457	2,196
Deferred income taxes	2,172		3,526	2,357
Uncertain tax positions	7,118		6,557	6,202
Other liabilities	$ 21,138		$ 20,538	$ 18,963