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Intangible Assets - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Goodwill And Intangible Assets Disclosure [Abstract]      
Amortization expense $ 900,000 $ 900,000 $ 900,000
Subsequent impairment of the underlying assets 0    
Expected annual amortization expense for 2019 900,000    
Expected annual amortization expense for 2020 100,000    
Expected annual amortization expense for 2021 100,000    
Expected annual amortization expense for 2022 100,000    
Expected annual amortization expense for 2023 100,000    
Expected annual amortization expense for 2024 100,000    
Expected annual amortization expense for 2025 $ 100,000