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Share-Based Compensation - Other Information Pertaining to Restricted Stock Units (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Restricted Stock Units (RSUs) [Member] | HCM Share Based Compensation Plans [Member]      
Weighted average grant date fair value per share $ 17.26 $ 17.40 $ 17.50