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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 receivable      
Deducted from asset accounts:      
Balance at Beginning of Period $ 857 $ 1,197 $ 1,578
Charged to Costs and Expenses 84 (34) 692
Charged to Other Accounts 0 0 0
Deduction 301 306 1,073
Balance at End of Period 640 857 1,197
Inventory reserves and obsolescence      
Deducted from asset accounts:      
Balance at Beginning of Period 14,407 7,869 8,193
Charged to Costs and Expenses 3,521 10,564 3,842
Charged to Other Accounts 0 0 0
Deduction 4,752 4,026 4,166
Balance at End of Period $ 13,176 $ 14,407 $ 7,869