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Taxation - Deferred tax assets and liabilities (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred income tax assets $ 43,437 $ 38,586  
Deferred income tax liabilities (10,457) (14,689)  
Deferred tax liability (asset) 32,980 23,897 $ 53,108
Deferred tax assets temporary differences      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred income tax assets 130,021 114,326 125,035
Deferred tax liability temporary differences      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred income tax liabilities (97,041) (90,429) $ (71,927)
Recovered after more than 12 months | Deferred tax assets temporary differences      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred income tax assets 109,830 96,822  
Recovered after more than 12 months | Deferred tax liability temporary differences      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred income tax liabilities (90,951) (86,573)  
Recovered within 12 months | Deferred tax assets temporary differences      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred income tax assets 20,191 17,504  
Recovered within 12 months | Deferred tax liability temporary differences      
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]      
Deferred income tax liabilities $ (6,090) $ (3,856)