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Financial instruments (Details 21) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Single series bond Two [Member] | Interest payment [Member]      
Financial instruments [Line Items]      
Repayments of bonds, notes and debentures $ 6,875 $ 13,750 $ 13,750