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Share-Based Compensation - Summary of Term-Based Option Activity (Detail) - Term Based Stock Options [Member] - HCM Share Based Compensation Plans [Member] - USD ($)
$ / shares in Units, $ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Shares          
Options Outstanding at Beginning of Period 10,994,181 9,769,770 7,607,158 7,588,078  
Options, Granted 5,140,787 2,285,981 2,339,238 187,140  
Options, Exercised (1,127,360) (595,464)      
Options, Forfeited or expired (166,747) (468,606) (176,626) (168,060)  
Options Outstanding at End of Period 14,840,861 10,994,181 9,769,770 7,607,158 7,588,078
Options Exercisable at End of Period 6,868,950 6,819,649      
Weighted Average Exercise Price (per share)          
Weighted Average Exercise Price at Beginning of Period $ 16.52 $ 16.20 $ 16.02 $ 15.98  
Weighted Average Exercise Price, Granted 22.84 17.46 16.80 17.56  
Weighted Average Exercise Price, Exercised (13.73) (15.14)      
Weighted Average Exercise Price, Forfeited or expired (17.09) (16.10) (16.54) (16.40)  
Weighted Average Exercise Price at End of Period 18.66 16.52 $ 16.20 $ 16.02 $ 15.98
Weighted Average Exercise Price Exercisable at End of Period $ 16.70 $ 16.14      
Weighted Average Remaining Contractual Term (in years)          
Weighted Average Remaining Contractual Term (in years) 7 years 7 months 6 days 6 years 10 months 24 days 7 years 1 month 6 days 7 years 3 months 18 days 8 years 2 months 12 days
Weighted Average Remaining Contractual Term Exercisable (in years) 5 years 9 months 18 days 5 years 6 months      
Aggregate intrinsic value          
Aggregate Intrinsic Value, Options Outstanding $ 346.8 $ 48.8 $ 9.9 $ 14.1 $ 13.3
Aggregate Intrinsic Value Exercisable at End of Period $ 174.0 $ 32.9      
Previously Reported [Member]          
Shares          
Options Outstanding at Beginning of Period 10,991,681        
Options Outstanding at End of Period   10,991,681