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Financial instruments (Details 15) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Series H bonds [Member] | Interest payment [Member]      
Financial instruments [Line Items]      
Repayments of bonds, notes and debentures $ 8,325 $ 7,691 $ 7,289