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Property and Equipment Net - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Property Plant And Equipment [Abstract]      
Property and equipment, depreciation and amortization expense $ 1,200,000 $ 1,100,000 $ 1,100,000
Impairment of long-lived assets $ 0 $ 150,000 $ 0