Adoption of Accounting Standards Codification 606 - Balance Sheet (Details) - USD ($) $ / shares in Units, $ in Thousands	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017	Apr. 01, 2017	Dec. 31, 2016
Current Assets
Cash and cash equivalents	$ 1,797		$ 2,150	$ 7,114	$ 7,432
Accounts receivable, net of allowance for doubtful accounts of $725 and $868 at March 31, 2018 and December 31, 2017, respectively	64,915		74,064
Contract assets	78,163	$ 68,739	0
Inventories	85,932	83,159	122,161
Production cost of contracts	11,181		11,204
Other current assets	12,503		11,435
Total Current Assets	254,491		221,014
Property and equipment, net of accumulated depreciation of $147,086 and $143,216 at March 31, 2018 and December 31, 2017, respectively	110,031		110,252
Goodwill	117,435		117,435
Intangibles, net	112,154		114,693
Non-current deferred income taxes	147	166	261
Other assets	3,311		3,098
Total Assets	597,569		566,753
Current Liabilities
Accounts payable	65,042		51,907
Contract liabilities	15,723	24,460	0
Accrued liabilities	22,469	22,238	28,329
Total Current Liabilities	103,234		80,236
Long-term debt	209,710		216,055
Non-current deferred income taxes	15,775	18,589	15,981
Other long-term liabilities	21,543		18,898
Total Liabilities	350,262		331,170
Commitments and contingencies (Notes 11, 13)
Equity [Abstract]
Common stock - $0.01 par value; 35,000,000 shares authorized; 11,380,982 and 11,332,841 shares issued and outstanding at March 31, 2018 and December 31, 2017, respectively	114		113
Additional paid-in capital	80,523		80,223
Retained earnings	173,652	170,029	161,364
Accumulated other comprehensive loss	(6,982)		(6,117)
Total Shareholders’ Equity	247,307		235,583
Total Liabilities and Shareholders’ Equity	597,569		566,753
Accounts receivable, allowance for doubtful accounts	725		868
Property and equipment, accumulated depreciation	$ 147,086		$ 143,216
Common stock, par value (in dollars per share)	$ 0.01		$ 0.01
Common stock, shares authorized (in shares)	35,000,000		35,000,000
Common stock, shares issued (in shares)	11,380,982		11,332,841
Common Stock, shares outstanding (in shares)	11,380,982		11,332,841
Calculated under Revenue Guidance in Effect before Topic 606
Current Assets
Cash and cash equivalents	$ 1,797		$ 2,150
Accounts receivable, net of allowance for doubtful accounts of $725 and $868 at March 31, 2018 and December 31, 2017, respectively	64,915
Contract assets	0		0
Inventories	137,850		122,161
Production cost of contracts	11,181
Other current assets	12,031
Total Current Assets	227,774
Property and equipment, net of accumulated depreciation of $147,086 and $143,216 at March 31, 2018 and December 31, 2017, respectively	110,031
Goodwill	117,435
Intangibles, net	112,154
Non-current deferred income taxes	261		261
Other assets	3,311
Total Assets	570,966
Current Liabilities
Accounts payable	65,042
Contract liabilities	0		0
Accrued liabilities	24,976		28,329
Total Current Liabilities	90,018
Long-term debt	209,710
Non-current deferred income taxes	12,659		15,981
Other long-term liabilities	21,543
Total Liabilities	333,930
Commitments and contingencies (Notes 11, 13)
Equity [Abstract]
Common stock - $0.01 par value; 35,000,000 shares authorized; 11,380,982 and 11,332,841 shares issued and outstanding at March 31, 2018 and December 31, 2017, respectively	114
Additional paid-in capital	80,523
Retained earnings	163,381		161,364
Accumulated other comprehensive loss	(6,982)
Total Shareholders’ Equity	237,036
Total Liabilities and Shareholders’ Equity	570,966
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Current Assets
Cash and cash equivalents	0		$ 0
Accounts receivable, net of allowance for doubtful accounts of $725 and $868 at March 31, 2018 and December 31, 2017, respectively	0
Contract assets	(78,163)	68,739
Inventories	51,918	(39,002)
Production cost of contracts	0
Other current assets	(472)
Total Current Assets	(26,717)
Property and equipment, net of accumulated depreciation of $147,086 and $143,216 at March 31, 2018 and December 31, 2017, respectively	0
Goodwill	0
Intangibles, net	0
Non-current deferred income taxes	114	(95)
Other assets	0
Total Assets	(26,603)
Current Liabilities
Accounts payable	0
Contract liabilities	(15,723)	24,460
Accrued liabilities	2,507	(6,091)
Total Current Liabilities	(13,216)
Long-term debt	0
Non-current deferred income taxes	(3,116)	2,608
Other long-term liabilities	0
Total Liabilities	(16,332)
Commitments and contingencies (Notes 11, 13)
Equity [Abstract]
Common stock - $0.01 par value; 35,000,000 shares authorized; 11,380,982 and 11,332,841 shares issued and outstanding at March 31, 2018 and December 31, 2017, respectively	0
Additional paid-in capital	0
Retained earnings	(10,271)	$ 8,665
Accumulated other comprehensive loss	0
Total Shareholders’ Equity	(10,271)
Total Liabilities and Shareholders’ Equity	$ (26,603)