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Commitments and Contingencies - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]      
Purchase order outstanding $ 6,528,000    
Total annual rental expense 6,738,000 $ 6,090,000 $ 5,778,000
Total annual rental income $ 1,474,000 $ 1,911,000 $ 1,921,000