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Intangible Assets - Schedule of Patent Cost and Other Asset (Detail) - Patents [Member] - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]    
Patent costs $ 2,093 $ 2,427
Accumulated amortization (1,386) (1,598)
Finite-lived intangible assets, net $ 707 $ 829