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Trade and Other Receivables - Ageing Analysis of Trade Receivables (Detail) - USD ($)
$ in Thousands
Jan. 01, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of financial assets [Line Items]      
Trade receivables $ 112,419 $ 112,403 $ 79,472
Neither past due nor impaired [Member]      
Disclosure of financial assets [Line Items]      
Trade receivables   90,503 59,959
Past due but not impaired [Member] | 1-3 months [Member]      
Disclosure of financial assets [Line Items]      
Trade receivables   20,608 16,858
Past due but not impaired [Member] | 3-6 months [Member]      
Disclosure of financial assets [Line Items]      
Trade receivables   654 1,642
Past due but not impaired [Member] | 6-12 months [Member]      
Disclosure of financial assets [Line Items]      
Trade receivables   397 474
Past due but not impaired [Member] | Over 12 months [Member]      
Disclosure of financial assets [Line Items]      
Trade receivables   $ 241 $ 539