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Goodwill and Intangible Assets - Schedule of Changes in Goodwill (Detail) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Goodwill [Line Items]      
Balance $ 1,961.0 $ 1,933.1  
Translation (11.2) 27.9  
Balance 1,949.8 1,961.0 $ 1,933.1
HCM [Member]      
Goodwill [Line Items]      
Balance $ 1,961.0 1,933.1 2,008.5
Formation of Joint Venture Company     (87.7)
Translation   27.9 12.3
Balance   1,961.0 $ 1,933.1
Tax-deductible goodwill   $ 12.2