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Accumulated other comprehensive income (loss) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Accumulated other comprehensive income (loss) [Roll Forward]      
Balance $ 2,847,246    
Other comprehensive income (loss) (5,976) $ (3,760) $ 6,951
Balance 3,079,105 2,847,246  
Net unrealized gain (loss) on derivative instruments qualifying as hedges      
Accumulated other comprehensive income (loss) [Roll Forward]      
Balance (1,430) (2,161)  
Other comprehensive income (loss) before reclassifications 0 0  
Amounts reclassified from accumulated other comprehensive loss 446 731  
Other comprehensive income (loss) 446 731  
Balance (984) (1,430) (2,161)
Postretirement liability adjustment      
Accumulated other comprehensive income (loss) [Roll Forward]      
Balance (40,734) (36,069)  
Other comprehensive income (loss) before reclassifications (8,395) (6,151)  
Amounts reclassified from accumulated other comprehensive loss 1,922 1,486  
Other comprehensive income (loss) (6,473) (4,665)  
Balance (47,207) (40,734) (36,069)
Net unrealized gain (loss) on available-for-sale investments      
Accumulated other comprehensive income (loss) [Roll Forward]      
Balance 62 (112)  
Other comprehensive income (loss) before reclassifications (1) 134  
Amounts reclassified from accumulated other comprehensive loss 52 40  
Other comprehensive income (loss) 51 174  
Balance 113 62 (112)
Total accumulated other comprehensive loss      
Accumulated other comprehensive income (loss) [Roll Forward]      
Balance (42,102) (38,342)  
Other comprehensive income (loss) before reclassifications (8,396) (6,017)  
Amounts reclassified from accumulated other comprehensive loss 2,420 2,257  
Other comprehensive income (loss) (5,976) (3,760) 6,951
Balance $ (48,078) $ (42,102) $ (38,342)