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Financial instruments - Non-Current Unsecured Interest-Bearing Bonds and Additional Information (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Oct. 31, 2018
Financial instruments [Line Items]      
Principal $ 345,949 $ 43,396  
Borrowing costs $ (2,360) $ (2,802)  
Single series bond Two      
Financial instruments [Line Items]      
Effective rate 1.17% 1.56%  
Nominal rate 4.38% 4.38%  
Principal $ 4,648 $ 4,648  
Borrowing costs $ (433) $ (433)  
Single series bond Three      
Financial instruments [Line Items]      
Effective rate 0.56% 0.74%  
Nominal rate 3.63% 3.63%  
Principal $ 302,658 $ 2,658  
Borrowing costs $ (170) $ (614)  
Single series bond Five      
Financial instruments [Line Items]      
Effective rate 3.01% 3.23%  
Nominal rate 4.25% 4.25%  
Principal $ 2,869 $ 2,869  
Borrowing costs $ (679) $ (679)  
Series H bonds      
Financial instruments [Line Items]      
Effective rate 1.23% 1.75%  
Nominal rate 4.90% 4.90%  
Principal $ 17,566 $ 16,026  
Borrowing costs $ (172) $ (172)  
Series O bonds      
Financial instruments [Line Items]      
Effective rate 1.89% 2.06%  
Nominal rate 3.80% 3.80%  
Principal $ 965 $ 863  
Borrowing costs $ (82) $ (82)  
Series P bonds      
Financial instruments [Line Items]      
Effective rate 1.72% 2.04%  
Nominal rate 3.25% 3.25%  
Principal $ 1,830 $ 1,636  
Borrowing costs $ (12) $ (12)  
Series Q bonds      
Financial instruments [Line Items]      
Effective rate 2.63% 2.72% 3.45%
Nominal rate 3.45% 3.45%  
Principal $ 351 $ 314  
Borrowing costs $ (20) $ (21)  
Single Series Bond Six      
Financial instruments [Line Items]      
Effective rate 3.79% 4.00%  
Nominal rate 4.25% 4.25%  
Principal $ 7,508 $ 7,508  
Borrowing costs $ (237) $ (237)  
Single Series Seventh Issue Bonds      
Financial instruments [Line Items]      
Effective rate 3.44% 3.62%  
Nominal rate 3.50% 3.50%  
Principal $ 7,554 $ 6,874  
Borrowing costs (555) (552)  
Unsecured obligations      
Financial instruments [Line Items]      
Principal 2,204,648 2,475,239  
Non-current maturities 2,204,648 2,475,239  
Borrowing costs (27,724) (30,020)  
Total 2,176,924 2,445,219  
Unsecured obligations | Over 1 year to 2 years      
Financial instruments [Line Items]      
Principal 0 300,000  
Non-current maturities 0 300,000  
Unsecured obligations | Over 2 year to 3 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Over 3 year to 4 years      
Financial instruments [Line Items]      
Principal 250,000 250,000  
Non-current maturities 250,000 250,000  
Unsecured obligations | Over 4 year to 5 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Over 5 Years      
Financial instruments [Line Items]      
Principal 1,954,648 1,925,239  
Non-current maturities $ 1,954,648 $ 1,925,239  
Unsecured obligations | Single series bond Two      
Financial instruments [Line Items]      
Effective rate 4.08% 4.08%  
Nominal rate 4.38% 4.38%  
Principal $ 250,000 $ 250,000  
Non-current maturities 250,000 250,000  
Borrowing costs (469) (903)  
Total 249,531 249,097  
Unsecured obligations | Single series bond Two | Over 1 year to 2 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Single series bond Two | Over 2 year to 3 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Single series bond Two | Over 3 year to 4 years      
Financial instruments [Line Items]      
Principal 250,000 250,000  
Non-current maturities 250,000 250,000  
Unsecured obligations | Single series bond Two | Over 4 year to 5 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Single series bond Two | Over 5 Years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities $ 0 $ 0  
Unsecured obligations | Single series bond Three      
Financial instruments [Line Items]      
Effective rate   3.42%  
Nominal rate   3.63%  
Principal   $ 300,000  
Non-current maturities   300,000  
Borrowing costs   (168)  
Total   299,832  
Unsecured obligations | Single series bond Three | Over 1 year to 2 years      
Financial instruments [Line Items]      
Principal   300,000  
Non-current maturities   300,000  
Unsecured obligations | Single series bond Three | Over 2 year to 3 years      
Financial instruments [Line Items]      
Principal   0  
Non-current maturities   0  
Unsecured obligations | Single series bond Three | Over 3 year to 4 years      
Financial instruments [Line Items]      
Principal   0  
Non-current maturities   0  
Unsecured obligations | Single series bond Three | Over 4 year to 5 years      
Financial instruments [Line Items]      
Principal   0  
Non-current maturities   0  
Unsecured obligations | Single series bond Three | Over 5 Years      
Financial instruments [Line Items]      
Principal   0  
Non-current maturities   $ 0  
Unsecured obligations | Single series bond Five      
Financial instruments [Line Items]      
Effective rate 4.10% 4.10%  
Nominal rate 4.25% 4.25%  
Principal $ 450,000 $ 450,000  
Non-current maturities 450,000 450,000  
Borrowing costs (3,666) (4,343)  
Total 446,334 445,657  
Unsecured obligations | Single series bond Five | Over 1 year to 2 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Single series bond Five | Over 2 year to 3 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Single series bond Five | Over 3 year to 4 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Single series bond Five | Over 4 year to 5 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Single series bond Five | Over 5 Years      
Financial instruments [Line Items]      
Principal 450,000 450,000  
Non-current maturities $ 450,000 $ 450,000  
Unsecured obligations | Series H bonds      
Financial instruments [Line Items]      
Effective rate 4.76% 4.76%  
Nominal rate 4.90% 4.90%  
Principal $ 96,967 $ 100,064  
Non-current maturities 96,967 100,064  
Borrowing costs (1,034) (1,206)  
Total 95,933 98,858  
Unsecured obligations | Series H bonds | Over 1 year to 2 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Series H bonds | Over 2 year to 3 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Series H bonds | Over 3 year to 4 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Series H bonds | Over 4 year to 5 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Series H bonds | Over 5 Years      
Financial instruments [Line Items]      
Principal 96,967 100,064  
Non-current maturities $ 96,967 $ 100,064  
Unsecured obligations | Series O bonds      
Financial instruments [Line Items]      
Effective rate 3.69% 3.69%  
Nominal rate 3.80% 3.80%  
Principal $ 61,536 $ 55,035  
Non-current maturities 61,536 55,035  
Borrowing costs (741) (822)  
Total 60,795 54,213  
Unsecured obligations | Series O bonds | Over 1 year to 2 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Series O bonds | Over 2 year to 3 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Series O bonds | Over 3 year to 4 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Series O bonds | Over 4 year to 5 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Series O bonds | Over 5 Years      
Financial instruments [Line Items]      
Principal 61,536 55,035  
Non-current maturities $ 61,536 $ 55,035  
Unsecured obligations | Series P bonds      
Financial instruments [Line Items]      
Effective rate 3.24% 3.24%  
Nominal rate 3.25% 3.25%  
Principal $ 123,072 $ 110,070  
Non-current maturities 123,072 110,070  
Borrowing costs (52) (65)  
Total 123,020 110,005  
Unsecured obligations | Series P bonds | Over 1 year to 2 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Series P bonds | Over 2 year to 3 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Series P bonds | Over 3 year to 4 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Series P bonds | Over 4 year to 5 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Series P bonds | Over 5 Years      
Financial instruments [Line Items]      
Principal 123,072 110,070  
Non-current maturities $ 123,072 $ 110,070  
Unsecured obligations | Series Q bonds      
Financial instruments [Line Items]      
Effective rate 3.43% 3.43%  
Nominal rate 3.45% 3.45%  
Principal $ 123,073 $ 110,070  
Non-current maturities 123,073 110,070  
Borrowing costs (309) (330)  
Total 122,764 109,740  
Unsecured obligations | Series Q bonds | Over 1 year to 2 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Series Q bonds | Over 2 year to 3 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Series Q bonds | Over 3 year to 4 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Series Q bonds | Over 4 year to 5 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Series Q bonds | Over 5 Years      
Financial instruments [Line Items]      
Principal 123,073 110,070  
Non-current maturities $ 123,073 $ 110,070  
Unsecured obligations | Single Series Bond Six      
Financial instruments [Line Items]      
Effective rate 4.19% 4.19%  
Nominal rate 4.25% 4.25%  
Principal $ 400,000 $ 400,000  
Non-current maturities 400,000 400,000  
Borrowing costs (6,112) (6,347)  
Total 393,888 393,653  
Unsecured obligations | Single Series Bond Six | Over 1 year to 2 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Single Series Bond Six | Over 2 year to 3 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Single Series Bond Six | Over 3 year to 4 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Single Series Bond Six | Over 4 year to 5 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Single Series Bond Six | Over 5 Years      
Financial instruments [Line Items]      
Principal 400,000 400,000  
Non-current maturities $ 400,000 $ 400,000  
Unsecured obligations | Single Series Seventh Issue Bonds      
Financial instruments [Line Items]      
Effective rate 3.42% 3.43%  
Nominal rate 3.50% 3.50%  
Principal $ 700,000 $ 700,000  
Non-current maturities 700,000 700,000  
Borrowing costs (15,341) (15,836)  
Total 684,659 684,164  
Unsecured obligations | Single Series Seventh Issue Bonds | Over 1 year to 2 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Single Series Seventh Issue Bonds | Over 2 year to 3 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Single Series Seventh Issue Bonds | Over 3 year to 4 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Single Series Seventh Issue Bonds | Over 4 year to 5 years      
Financial instruments [Line Items]      
Principal 0 0  
Non-current maturities 0 0  
Unsecured obligations | Single Series Seventh Issue Bonds | Over 5 Years      
Financial instruments [Line Items]      
Principal 700,000 700,000  
Non-current maturities $ 700,000 $ 700,000