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Business Combinations and Noncontrolling Interest - Additional Information (Detail) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Mar. 01, 2016
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Dec. 31, 2016
Business Combinations [Line Items]          
Non-controlling interest $ 39.2     $ 37.8 $ 38.7
Net assets acquired $ 2.1        
Number of Class B shares exchanged 10,063,749        
Cash acquired $ 1.2        
Net wokring capital acquired 0.9        
Goodwill related to the acquisition price $ 37.1        
Net loss attributable to noncontrolling interest   $ (0.5) $ (0.4) (1.3) 0.1
Non-controlling Interest          
Business Combinations [Line Items]          
Net loss attributable to noncontrolling interest       $ 1.3 $ 0.1