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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
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Shares    
Options Outstanding at Beginning of Period 1,038,147  
Options, Granted 0  
Options, Exercised (546,609)  
Options, Forfeited or expired (8,358)  
Options Outstanding at End of Period 483,180 1,038,147
Options Exercisable at End of Period 483,180  
Weighted Average Exercise Price (per share)    
Weighted Average Exercise Price at Beginning of Period $ 13.46  
Weighted Average Exercise Price, Granted 0  
Weighted Average Exercise Price, Exercised (13.46)  
Weighted Average Exercise Price, Forfeited or expired (13.46)  
Weighted Average Exercise Price at End of Period 13.49 $ 13.46
Weighted Average Exercise Price at End of Period $ 13.49  
Weighted Average Remaining Contractual Term (in years)    
Weighted Average Remaining Contractual Term (in years) 3 years 4 months 24 days 3 years 6 months
Performance-based options exercisable at June 30, 2018 3 years 4 months 24 days  
Aggregate intrinsic value    
Aggregate Intrinsic Value $ 9.5  
Aggregate Intrinsic Value 9.5  
Aggregate Intrinsic Value $ 9.5