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Share-based Payment - Number and Weighted Average Exercise Prices ("WAEP") of, and Movements in Share Options (Detail)
12 Months Ended
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Disclosure of terms and conditions of share-based payment arrangement [abstract]    
Beginning balance 470,000 530,000
Exercised during the year 0 (46,400)
Cancelled during the year   (13,600)
Ending balance 470,000 470,000
Exercisable at December 31 470,000 470,000
Beginning balance $ 21.11 $ 21.11
Exercised during the year 21.11 21.11
Cancelled during the year 21.11 21.11
Ending balance 21.11 21.11
Exercisable at December 31 $ 21.11 $ 21.11