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Share-Based Compensation - Summary of Performance Based Stock Option (Detail) - Performance Shares [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 1,038,147 1,227,926 1,280,868 1,343,100  
Options, Granted 0 0 0 0  
Options, Exercised (633,606) (167,202)      
Options, Forfeited or expired (8,358) (25,077) (52,942) (62,232)  
Options Outstanding at End of Period 396,183 1,038,147 1,227,926 1,280,868 1,343,100
Options Exercisable at End of Period 396,183        
Weighted Average Exercise Price (per share)          
Weighted Average Exercise Price at Beginning of Period $ 13.46 $ 13.46 $ 13.46 $ 13.46  
Weighted Average Exercise Price, Granted 0 0 0 0  
Weighted Average Exercise Price, Exercised (13.46) (13.46)      
Weighted Average Exercise Price, Forfeited or expired (13.46) (13.46) (13.46) (13.46)  
Weighted Average Exercise Price at End of Period 13.50 $ 13.46 $ 13.46 $ 13.46 $ 13.46
Weighted Average Exercise Price at End of Period $ 13.50        
Weighted Average Remaining Contractual Term (in years)          
Weighted Average Remaining Contractual Term (in years) 3 years 4 months 24 days 3 years 6 months 4 years 2 months 12 days 5 years 2 months 12 days 6 years 3 months 18 days
Weighted Average Remaining Contractual Term Exercisable (in years) 3 years 4 months 24 days        
Aggregate intrinsic value          
Aggregate Intrinsic Value, Options Outstanding $ 11.3        
Aggregate Intrinsic Value, Options Exercisable $ 11.3        
Previously Reported [Member]          
Shares          
Options Outstanding at Beginning of Period 1,035,647        
Options Outstanding at End of Period   1,035,647