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Valuation and Qualifying Accounts (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Allowance for Doubtful Accounts [Member]      
Valuation and Qualifying Accounts Disclosure [Line Items]      
Balance at Beginning of Period $ 22 $ 82 $ 188
Additions Charged to Costs and Expenses 34
Deductions, Accounts Charged Off and Recoveries of Amounts Previously Written Off   (60) (106)
Balance at End of Period 56 22 82
Valuation Allowance of Deferred Tax Assets [Member]      
Valuation and Qualifying Accounts Disclosure [Line Items]      
Balance at Beginning of Period 5,197 5,797 6,219
Additions Charged to Costs and Expenses
Deductions, Accounts Charged Off and Recoveries of Amounts Previously Written Off 1,066 (600) (422)
Balance at End of Period 6,263 5,197 5,797
Inventory Valuation Reserve [Member]      
Valuation and Qualifying Accounts Disclosure [Line Items]      
Balance at Beginning of Period 4,307 3,793 2,381
Additions Charged to Costs and Expenses 2,672 2,344 1,458
Deductions, Accounts Charged Off and Recoveries of Amounts Previously Written Off (819) (1,830) (46)
Balance at End of Period $ 6,160 $ 4,307 $ 3,793