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Property, Plant, and Equipment - Additional Information (Details) - 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 [Abstract]            
Depreciation expense $ 100 $ 100 $ 122 $ 236 $ 444 $ 310