Selected Balance Sheet Information (Details) (USD $) In Thousands, unless otherwise specified	Jul. 31, 2013	Apr. 30, 2013	Jan. 31, 2013	Jul. 31, 2012	Apr. 30, 2012	Jan. 31, 2012
Accounts receivable, net:
Trade accounts	$ 49,404		$ 56,508	$ 50,055
Allowance for doubtful accounts	(205)		(205)	(170)
Accounts receivable, net	49,199		56,303	49,885
Inventories:
Finished goods	7,980		8,571	7,394
In process	2,562		2,675	4,979
Materials	43,820		34,943	38,016
Inventories	54,362		46,189	50,389
Property, plant and equipment, net:
Property, plant and equipment	171,392		156,658	145,433
Accumulated depreciation	(81,127)		(75,420)	(72,244)
Property, plant and equipment, net	90,265		81,238	73,189
Accrued liabilities:
Salaries and benefits	2,391		4,265	3,605
Vacation	4,171		4,025	4,155
401(k) contributions	417		520	611
Insurance obligations	2,321		2,506	3,003
Warranties	2,047	1,990	1,888	1,949	1,792	1,699
Acquisition-related contingent liabilities	594		712	7,028
Taxes - accrued and withheld	1,900		1,392	1,999
Other	1,450		1,884	1,621
Accrued liabilities	15,291		17,192	23,971
Other liabilities:
Postretirement benefits	8,208		8,072	7,520
Acquisition-related contingent consideration	2,196		2,359	2,108
Deferred income taxes	2,357		2,453	3,919
Uncertain tax positions	6,202		5,818	5,657
Other liabilities	$ 18,963		$ 18,702	$ 19,204