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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 $ 46 $ 42 $ 423
Additions Charged to Costs and Expenses 1,217 31 3
Deductions   (27) (384)
Balance at End of Period 1,263 46 42
Inventory Reserve      
Valuation And Qualifying Accounts Disclosure [Line Items]      
Balance at Beginning of Period 3,137 3,594 4,020
Additions Charged to Costs and Expenses 476    
Deductions (1,624) (457) (426)
Balance at End of Period $ 1,989 $ 3,137 $ 3,594