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Accounts receivable, net - Additional Information (Detail) - Customer
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Number of top customers 10  
Credit concentration risk [Member] | Accounts receivable [Member]    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Accounts receivable, percentage 86.00% 76.00%