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Trade and other receivables (Narrative) (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Jan. 01, 2017
Statements [Line Items]      
Statutory receivables $ 12,764 $ 13,961 $ 43,808
Peru [Member]      
Statements [Line Items]      
Statutory receivables $ 11,670 $ 10,905 $ 42,273