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Financial instruments - Unsecured Obligations, Current (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Oct. 31, 2018
Financial instruments [Line Items]      
Notional amounts $ 345,949 $ 43,396  
Current maturities 345,949 43,396  
Borrowing costs (2,360) (2,802)  
Total 343,589 40,594  
Not later than three months      
Financial instruments [Line Items]      
Notional amounts 32,517 37,555  
Current maturities 32,517 37,555  
Later than three months and not later than one year      
Financial instruments [Line Items]      
Notional amounts 313,432 5,841  
Current maturities $ 313,432 $ 5,841  
Single series bond Two      
Financial instruments [Line Items]      
Nominal rate 4.38% 4.38%  
Effective rate 1.17% 1.56%  
Notional amounts $ 4,648 $ 4,648  
Current maturities 4,648 4,648  
Borrowing costs (433) (433)  
Total 4,215 4,215  
Single series bond Two | Not later than three months      
Financial instruments [Line Items]      
Notional amounts 4,648 4,648  
Current maturities 4,648 4,648  
Single series bond Two | Later than three months and not later than one year      
Financial instruments [Line Items]      
Notional amounts 0 0  
Current maturities $ 0 $ 0  
Single series bond Three      
Financial instruments [Line Items]      
Nominal rate 3.63% 3.63%  
Effective rate 0.56% 0.74%  
Notional amounts $ 302,658 $ 2,658  
Current maturities 302,658 2,658  
Borrowing costs (170) (614)  
Total 302,488 2,044  
Single series bond Three | Not later than three months      
Financial instruments [Line Items]      
Notional amounts 0 0  
Current maturities 0 0  
Single series bond Three | Later than three months and not later than one year      
Financial instruments [Line Items]      
Notional amounts 302,658 2,658  
Current maturities $ 302,658 $ 2,658  
Single series bond Five      
Financial instruments [Line Items]      
Nominal rate 4.25% 4.25%  
Effective rate 3.01% 3.23%  
Notional amounts $ 2,869 $ 2,869  
Current maturities 2,869 2,869  
Borrowing costs (679) (679)  
Total 2,190 2,190  
Single series bond Five | Not later than three months      
Financial instruments [Line Items]      
Notional amounts 0 0  
Current maturities 0 0  
Single series bond Five | Later than three months and not later than one year      
Financial instruments [Line Items]      
Notional amounts 2,869 2,869  
Current maturities $ 2,869 $ 2,869  
Series H bonds      
Financial instruments [Line Items]      
Nominal rate 4.90% 4.90%  
Effective rate 1.23% 1.75%  
Notional amounts $ 17,566 $ 16,026  
Current maturities 17,566 16,026  
Borrowing costs (172) (172)  
Total 17,394 15,854  
Series H bonds | Not later than three months      
Financial instruments [Line Items]      
Notional amounts 17,566 16,026  
Current maturities 17,566 16,026  
Series H bonds | Later than three months and not later than one year      
Financial instruments [Line Items]      
Notional amounts 0 0  
Current maturities $ 0 $ 0  
Series O bonds      
Financial instruments [Line Items]      
Nominal rate 3.80% 3.80%  
Effective rate 1.89% 2.06%  
Notional amounts $ 965 $ 863  
Current maturities 965 863  
Borrowing costs (82) (82)  
Total 883 781  
Series O bonds | Not later than three months      
Financial instruments [Line Items]      
Notional amounts 965 863  
Current maturities 965 863  
Series O bonds | Later than three months and not later than one year      
Financial instruments [Line Items]      
Notional amounts 0 0  
Current maturities $ 0 $ 0  
Series P bonds      
Financial instruments [Line Items]      
Nominal rate 3.25% 3.25%  
Effective rate 1.72% 2.04%  
Notional amounts $ 1,830 $ 1,636  
Current maturities 1,830 1,636  
Borrowing costs (12) (12)  
Total 1,818 1,624  
Series P bonds | Not later than three months      
Financial instruments [Line Items]      
Notional amounts 1,830 1,636  
Current maturities 1,830 1,636  
Series P bonds | Later than three months and not later than one year      
Financial instruments [Line Items]      
Notional amounts 0 0  
Current maturities $ 0 $ 0  
Series Q bonds      
Financial instruments [Line Items]      
Nominal rate 3.45% 3.45%  
Effective rate 2.63% 2.72% 3.45%
Notional amounts $ 351 $ 314  
Current maturities 351 314  
Borrowing costs (20) (21)  
Total 331 293  
Series Q bonds | Not later than three months      
Financial instruments [Line Items]      
Notional amounts 0 0  
Current maturities 0 0  
Series Q bonds | Later than three months and not later than one year      
Financial instruments [Line Items]      
Notional amounts 351 314  
Current maturities $ 351 $ 314  
Single Series Bond Six      
Financial instruments [Line Items]      
Nominal rate 4.25% 4.25%  
Effective rate 3.79% 4.00%  
Notional amounts $ 7,508 $ 7,508  
Current maturities 7,508 7,508  
Borrowing costs (237) (237)  
Total 7,271 7,271  
Single Series Bond Six | Not later than three months      
Financial instruments [Line Items]      
Notional amounts 7,508 7,508  
Current maturities 7,508 7,508  
Single Series Bond Six | Later than three months and not later than one year      
Financial instruments [Line Items]      
Notional amounts 0 0  
Current maturities $ 0 $ 0  
Single Series Seventh Issue Bonds      
Financial instruments [Line Items]      
Nominal rate 3.50% 3.50%  
Effective rate 3.44% 3.62%  
Notional amounts $ 7,554 $ 6,874  
Current maturities 7,554 6,874  
Borrowing costs (555) (552)  
Total 6,999 6,322  
Single Series Seventh Issue Bonds | Not later than three months      
Financial instruments [Line Items]      
Notional amounts 0 6,874  
Current maturities 0 6,874  
Single Series Seventh Issue Bonds | Later than three months and not later than one year      
Financial instruments [Line Items]      
Notional amounts 7,554 0  
Current maturities $ 7,554 $ 0