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Financial instruments and risk management - Credit risk - Additional information (Detail) - Credit risk [Member] - Customer
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Disclosure Of Financial Instruments [Line Items]    
Description of customers receivable owing The Group had approximately [XX] customers (2016: 92 customers) that owed the Group more than US$5 million each and these balances accounted for approximately [XX] per cent (2016: 74 per cent) of all receivables owing. There were [XX] customers (2016: 24 customers) with balances greater than US$20 million accounting for just over [XX] per cent (2016: 42 per cent) of total amounts receivable.  
More than US dollar 5 million [member]    
Disclosure Of Financial Instruments [Line Items]    
Number of customers 115 92
Per cent of receivables 78.00% 74.00%
Greater than US dollar 20 million [member]    
Disclosure Of Financial Instruments [Line Items]    
Number of customers 23 24
Per cent of receivables 35.00% 42.00%