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Properties, Plant and Equipment - Additional Information (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Property, Plant and Equipment [Abstract]        
Depreciation expense $ 0.8 $ 1.0 $ 1.6 $ 2.1