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Intangible Assets - Additional Information (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Finite-Lived Intangible Assets [Line Items]      
Amortization expense $ 130,000 $ 134,000 $ 145,000
Patents [Member]      
Finite-Lived Intangible Assets [Line Items]      
Future amortization expense from patent assets held for 2018 112,000    
Future amortization expense from patent assets held for 2019 107,000    
Future amortization expense from patent assets held for 2020 102,000    
Future amortization expense from patent assets held for 2021 93,000    
Future amortization expense from patent assets held for 2022 $ 61,000