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Restricted Stock Units - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Employee service share-based compensation, nonvested awards, compensation cost not yet recognized, period for recognition 2 years 6 months    
Nonvested Restricted Stock Units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Deferred compensation liability, Current, total $ 1,751,000 $ 1,463,000 $ 816,000
Employee service share-based compensation, nonvested awards, compensation not yet recognized, share-based awards other than options $ 3,200,000    
Employee service share-based compensation, nonvested awards, compensation cost not yet recognized, period for recognition 2 years 3 months 18 days    
Share based compensation arrangement by share based payment award non option equity instruments grants in period weighted average grant date fair value $ 0.82 $ 3.74 $ 2.94