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Financial instruments - Unsecured Obligations, Non-Current (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Financial instruments [Line Items]    
Notional amount $ 50,707 $ 345,949
Bond issuance costs (3,708) (2,360)
Unsecured obligations    
Financial instruments [Line Items]    
Notional amount 2,948,462 2,204,648
Carrying amount 2,948,462 2,204,648
Bond issuance costs (38,977) (27,724)
Total 2,909,485 2,176,924
Unsecured obligations | Over 1 year to 2 years    
Financial instruments [Line Items]    
Notional amount 250,000 0
Carrying amount 250,000 0
Unsecured obligations | Over 2 years to 3 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Unsecured obligations | Over 3 years to 4 years    
Financial instruments [Line Items]    
Notional amount 0 250,000
Carrying amount 0 250,000
Unsecured obligations | Over 4 years to 5 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Unsecured obligations | Over 5 years    
Financial instruments [Line Items]    
Notional amount 2,698,462 1,954,648
Carrying amount $ 2,698,462 $ 1,954,648
Single Series Bond One    
Financial instruments [Line Items]    
Effective rate 0.80% 1.17%
Nominal rate 4.38% 4.38%
Notional amount $ 4,648 $ 4,648
Bond issuance costs $ (433) $ (433)
Single Series Bond One | Unsecured obligations    
Financial instruments [Line Items]    
Effective rate 4.24% 4.08%
Nominal rate 4.38% 4.38%
Notional amount $ 250,000 $ 250,000
Carrying amount 250,000 250,000
Bond issuance costs (36) (469)
Total 249,964 249,531
Single Series Bond One | Unsecured obligations | Over 1 year to 2 years    
Financial instruments [Line Items]    
Notional amount 250,000 0
Carrying amount 250,000 0
Single Series Bond One | Unsecured obligations | Over 2 years to 3 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Single Series Bond One | Unsecured obligations | Over 3 years to 4 years    
Financial instruments [Line Items]    
Notional amount 0 250,000
Carrying amount 0 250,000
Single Series Bond One | Unsecured obligations | Over 4 years to 5 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Single Series Bond One | Unsecured obligations | Over 5 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount $ 0 $ 0
Single Series Bond Two    
Financial instruments [Line Items]    
Effective rate 2.39% 0.56%
Nominal rate 4.25% 3.63%
Notional amount $ 2,869 $ 302,658
Bond issuance costs $ (677) $ (170)
Single Series Bond Two | Unsecured obligations    
Financial instruments [Line Items]    
Effective rate 4.14% 4.10%
Nominal rate 4.25% 4.25%
Notional amount $ 450,000 $ 450,000
Carrying amount 450,000 450,000
Bond issuance costs (2,991) (3,666)
Total 447,009 446,334
Single Series Bond Two | Unsecured obligations | Over 1 year to 2 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Single Series Bond Two | Unsecured obligations | Over 2 years to 3 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Single Series Bond Two | Unsecured obligations | Over 3 years to 4 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Single Series Bond Two | Unsecured obligations | Over 4 years to 5 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Single Series Bond Two | Unsecured obligations | Over 5 years    
Financial instruments [Line Items]    
Notional amount 450,000 450,000
Carrying amount $ 450,000 $ 450,000
Single Series Bond Three    
Financial instruments [Line Items]    
Effective rate 3.62% 3.01%
Nominal rate 4.25% 4.25%
Notional amount $ 7,508 $ 2,869
Bond issuance costs $ (235) $ (679)
Single Series Bond Three | Unsecured obligations    
Financial instruments [Line Items]    
Effective rate 4.23% 4.19%
Nominal rate 4.25% 4.25%
Notional amount $ 400,000 $ 400,000
Carrying amount 400,000 400,000
Bond issuance costs (5,879) (6,112)
Total 394,121 393,888
Single Series Bond Three | Unsecured obligations | Over 1 year to 2 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Single Series Bond Three | Unsecured obligations | Over 2 years to 3 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Single Series Bond Three | Unsecured obligations | Over 3 years to 4 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Single Series Bond Three | Unsecured obligations | Over 4 years to 5 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Single Series Bond Three | Unsecured obligations | Over 5 years    
Financial instruments [Line Items]    
Notional amount 400,000 400,000
Carrying amount $ 400,000 $ 400,000
Single Series Bond Four    
Financial instruments [Line Items]    
Effective rate 3.30% 3.79%
Nominal rate 3.50% 4.25%
Notional amount $ 7,554 $ 7,508
Bond issuance costs $ (555) $ (237)
Single Series Bond Four | Unsecured obligations    
Financial instruments [Line Items]    
Effective rate 3.45% 3.42%
Nominal rate 3.50% 3.50%
Notional amount $ 700,000 $ 700,000
Carrying amount 700,000 700,000
Bond issuance costs (14,787) (15,341)
Total 685,213 684,659
Single Series Bond Four | Unsecured obligations | Over 1 year to 2 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Single Series Bond Four | Unsecured obligations | Over 2 years to 3 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Single Series Bond Four | Unsecured obligations | Over 3 years to 4 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Single Series Bond Four | Unsecured obligations | Over 4 years to 5 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Single Series Bond Four | Unsecured obligations | Over 5 years    
Financial instruments [Line Items]    
Notional amount 700,000 700,000
Carrying amount $ 700,000 $ 700,000
Single Series Bond Five    
Financial instruments [Line Items]    
Effective rate 6.89% 3.44%
Nominal rate 6.50% 3.50%
Notional amount $ 7,312 $ 7,554
Bond issuance costs $ (1,521) $ (555)
Single Series Bond Five | Unsecured obligations    
Financial instruments [Line Items]    
Effective rate 6.89%  
Nominal rate 6.50%  
Notional amount $ 750,000  
Carrying amount 750,000  
Bond issuance costs (13,437)  
Total 736,563  
Single Series Bond Five | Unsecured obligations | Over 1 year to 2 years    
Financial instruments [Line Items]    
Notional amount 0  
Carrying amount 0  
Single Series Bond Five | Unsecured obligations | Over 2 years to 3 years    
Financial instruments [Line Items]    
Notional amount 0  
Carrying amount 0  
Single Series Bond Five | Unsecured obligations | Over 3 years to 4 years    
Financial instruments [Line Items]    
Notional amount 0  
Carrying amount 0  
Single Series Bond Five | Unsecured obligations | Over 4 years to 5 years    
Financial instruments [Line Items]    
Notional amount 0  
Carrying amount 0  
Single Series Bond Five | Unsecured obligations | Over 5 years    
Financial instruments [Line Items]    
Notional amount 750,000  
Carrying amount $ 750,000  
Series H bonds    
Financial instruments [Line Items]    
Effective rate 1.58% 1.23%
Nominal rate 4.90% 4.90%
Notional amount $ 17,599 $ 17,566
Bond issuance costs $ (172) $ (172)
Series H bonds | Unsecured obligations    
Financial instruments [Line Items]    
Effective rate 4.76% 4.76%
Nominal rate 4.90% 4.90%
Notional amount $ 83,887 $ 96,967
Carrying amount 83,887 96,967
Bond issuance costs (861) (1,034)
Total 83,026 95,933
Series H bonds | Unsecured obligations | Over 1 year to 2 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Series H bonds | Unsecured obligations | Over 2 years to 3 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Series H bonds | Unsecured obligations | Over 3 years to 4 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Series H bonds | Unsecured obligations | Over 4 years to 5 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Series H bonds | Unsecured obligations | Over 5 years    
Financial instruments [Line Items]    
Notional amount 83,887 96,967
Carrying amount $ 83,887 $ 96,967
Series O bonds    
Financial instruments [Line Items]    
Effective rate 1.68% 1.89%
Nominal rate 3.80% 3.80%
Notional amount $ 987 $ 965
Bond issuance costs $ (82) $ (82)
Series O bonds | Unsecured obligations    
Financial instruments [Line Items]    
Effective rate 3.69% 3.69%
Nominal rate 3.80% 3.80%
Notional amount $ 62,915 $ 61,536
Carrying amount 62,915 61,536
Bond issuance costs (659) (741)
Total 62,256 60,795
Series O bonds | Unsecured obligations | Over 1 year to 2 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Series O bonds | Unsecured obligations | Over 2 years to 3 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Series O bonds | Unsecured obligations | Over 3 years to 4 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Series O bonds | Unsecured obligations | Over 4 years to 5 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Series O bonds | Unsecured obligations | Over 5 years    
Financial instruments [Line Items]    
Notional amount 62,915 61,536
Carrying amount $ 62,915 $ 61,536
Series P bonds    
Financial instruments [Line Items]    
Effective rate 1.41% 1.72%
Nominal rate 3.25% 3.25%
Notional amount $ 1,871 $ 1,830
Bond issuance costs $ (12) $ (12)
Series P bonds | Unsecured obligations    
Financial instruments [Line Items]    
Effective rate 3.24% 3.24%
Nominal rate 3.25% 3.25%
Notional amount $ 125,830 $ 123,072
Carrying amount 125,830 123,072
Bond issuance costs (40) (52)
Total 125,790 123,020
Series P bonds | Unsecured obligations | Over 1 year to 2 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Series P bonds | Unsecured obligations | Over 2 years to 3 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Series P bonds | Unsecured obligations | Over 3 years to 4 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Series P bonds | Unsecured obligations | Over 4 years to 5 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Series P bonds | Unsecured obligations | Over 5 years    
Financial instruments [Line Items]    
Notional amount 125,830 123,072
Carrying amount $ 125,830 $ 123,072
Series Q bonds    
Financial instruments [Line Items]    
Effective rate 2.41% 2.63%
Nominal rate 3.45% 3.45%
Notional amount $ 359 $ 351
Bond issuance costs $ (21) $ (20)
Series Q bonds | Unsecured obligations    
Financial instruments [Line Items]    
Effective rate 3.54% 3.43%
Nominal rate 3.45% 3.45%
Notional amount $ 125,830 $ 123,073
Carrying amount 125,830 123,073
Bond issuance costs (287) (309)
Total 125,543 122,764
Series Q bonds | Unsecured obligations | Over 1 year to 2 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Series Q bonds | Unsecured obligations | Over 2 years to 3 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Series Q bonds | Unsecured obligations | Over 3 years to 4 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Series Q bonds | Unsecured obligations | Over 4 years to 5 years    
Financial instruments [Line Items]    
Notional amount 0 0
Carrying amount 0 0
Series Q bonds | Unsecured obligations | Over 5 years    
Financial instruments [Line Items]    
Notional amount 125,830 123,073
Carrying amount $ 125,830 $ 123,073