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Property, Plant and Equipment - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Property Plant And Equipment Capitalized Interest Costs [Abstract]            
Depreciation expense $ 2,294 $ 2,215 $ 4,585 $ 4,383 $ 8,822 $ 6,992