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Property, Plant, and Equipment - Additional Information (Detail) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Property, Plant and Equipment [Abstract]          
Depreciation expense $ 28.4 $ 25.8 $ 35.3 $ 34.8 $ 33.5