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Product Warranties - Product Warranty Activity (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Guarantees [Abstract]      
Balance at the beginning of the period $ 214 $ 585 $ 204
Accruals for warranties for products sold in the period 346 358 715
Fulfillment of warranty obligations (194) (527) (334)
Revisions of estimated obligations (76) (202)  
Balance at the end of the period $ 290 $ 214 $ 585