New Pronouncements Adopted (Details 1) - USD ($) $ in Thousands	Mar. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Retained Earnings Adjustments [Line Items]
Unbilled receivables		$ 2,658	$ 0
Inventories	$ 216,443	218,293	220,352
Other liabilities and accrued items	27,423	28,105	28,044
Deferred income taxes	218	326	213
Retained earnings	$ 545,093	536,541	$ 536,116
Adjustments for New Accounting Pronouncement [Member]
Retained Earnings Adjustments [Line Items]
Unbilled receivables		2,658
Inventories		(2,059)
Other liabilities and accrued items		61
Deferred income taxes		113
Retained earnings		$ 425