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Accumulated other comprehensive loss (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2023
Mar. 31, 2022
Accumulated other comprehensive loss [Roll Forward]    
Balance $ 3,587,129 $ 3,382,874
Other comprehensive income (loss) 217 269
Balance 3,575,382 3,360,718
Net unrealized gain (loss) on derivative instruments qualifying as hedges    
Accumulated other comprehensive loss [Roll Forward]    
Balance (125) (538)
Other comprehensive loss before reclassifications 0 0
Amounts reclassified from accumulated other comprehensive loss 34 112
Other comprehensive income (loss) 34 112
Balance (91) (426)
Postretirement liability adjustment    
Accumulated other comprehensive loss [Roll Forward]    
Balance (29,900) (40,461)
Other comprehensive loss before reclassifications 0 0
Amounts reclassified from accumulated other comprehensive loss 100 445
Other comprehensive income (loss) 100 445
Balance (29,800) (40,016)
Net unrealized gain (loss) on available-for-sale investments    
Accumulated other comprehensive loss [Roll Forward]    
Balance (558) (5)
Other comprehensive loss before reclassifications 70 (320)
Amounts reclassified from accumulated other comprehensive loss 13 32
Other comprehensive income (loss) 83 (288)
Balance (475) (293)
Total accumulated other comprehensive loss    
Accumulated other comprehensive loss [Roll Forward]    
Balance (30,583) (41,004)
Other comprehensive loss before reclassifications 70 (320)
Amounts reclassified from accumulated other comprehensive loss 147 589
Other comprehensive income (loss) 217 269
Balance $ (30,366) $ (40,735)