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Related Party Transactions - Additional Information (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of transactions between related parties [line items]      
Non-executive directors' fees paid $ 1.2 $ 1.0 $ 0.8
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 0.20% 0.20%