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Valuation and Qualifying Accounts (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Allowance for Doubtful Accounts      
Valuation And Qualifying Accounts Disclosure [Line Items]      
Balance at Beginning of Period $ 42 $ 423 $ 166
Additions Charged to Costs and Expenses 31 3 332
Deductions (27) (384) (75)
Balance at End of Period 46 42 423
Inventory Reserve      
Valuation And Qualifying Accounts Disclosure [Line Items]      
Balance at Beginning of Period 3,594 4,020 2,995
Additions Charged to Costs and Expenses     1,025
Deductions (457) (426)  
Balance at End of Period $ 3,137 $ 3,594 $ 4,020