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Share-Based Compensation - Other Information Pertaining to Term-Based Options (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Term Based Stock Options [Member] | HCM Share Based Compensation Plans [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted average grant date fair value per share $ 5.88 $ 5.74 $ 5.68