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Financial instruments (Details 23) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Series Q bonds [Member] | Interest payment [Member]      
Financial instruments [Line Items]      
Repayments of bonds, notes and debentures $ 3,769 $ 3,791 $ 319
Series Q bonds [Member] | CCS Coverage [Member]      
Financial instruments [Line Items]      
Repayments of bonds, notes and debentures 1,021 0 0
Single Series Fifth Issue Bonds [Member] | Interest payment [Member]      
Financial instruments [Line Items]      
Repayments of bonds, notes and debentures 19,125 9,563 0
Single Series Sixth Issue Bonds [Member] | Interest payment [Member]      
Financial instruments [Line Items]      
Repayments of bonds, notes and debentures $ 8,500 $ 0 $ 0