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Property and Equipment - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Property, Plant and Equipment [Abstract]      
Depreciation $ 2,400,000 $ 1,500,000 $ 1,000,000
Gain on disposal of capital equipment $ 7,000 $ 8,000 $ 9,000