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Financial instruments and risk management - Credit risk - Additional information (Detail) - Credit risk [Member] - Customer
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Disclosure Of Financial Instruments [Line Items]    
Description of customers receivable owing The Group had approximately 113 customers (2017: 115 customers) that owed the Group more than US$5 million each and these balances accounted for approximately 84% (2017: 78%) of all trade receivables. There were 17 customers (2017: 23 customers) with balances greater than US$20 million accounting for just over 34% (2017: 35%) of all trade receivables.  
More than US dollar 5 million [member]    
Disclosure Of Financial Instruments [Line Items]    
Number of customers 113 115
Per cent of trade receivables 84.00% 78.00%
Greater than US dollar 20 million [member]    
Disclosure Of Financial Instruments [Line Items]    
Number of customers 17 23
Per cent of trade receivables 34.00% 35.00%