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Allowance for Doubtful Accounts, Sales Returns and Discounts (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Balance at beginning of year $ 1,395 $ 775 $ 727
Charges to earnings 155 620 310
Amounts utilized (1,550) 0 (262)
Balance at end of year $ 0 $ 1,395 $ 775