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Related Party Transactions - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure of transactions between related parties [line items]    
Key management remuneration $ 17.0 $ 17.0
Director's interest in issued share capital or voting control 1.00%  
Executive Committee and Non-Executive Directors [Member]    
Disclosure of transactions between related parties [line items]    
Key management personnel's beneficial interest in issued and listed share capital 0.10% 0.20%