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Pension and postretirement benefits - Components of net periodic benefit cost (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Pension Plans        
Personnel costs:        
Service cost $ 0 $ 0 $ 0 $ 0
Other operating expenses        
Interest cost 3,998 5,847 7,996 11,694
Expected return on plan assets (9,670) (9,526) (19,341) (19,052)
Amortization of prior service cost (credit) 0 0 0 0
Amortization of net loss 4,720 5,220 9,440 10,440
Total net periodic pension cost (952) 1,541 (1,905) 3,082
OPEB Plan        
Personnel costs:        
Service cost 160 178 320 356
Other operating expenses        
Interest cost 893 1,228 1,785 2,456
Expected return on plan assets 0 0 0 0
Amortization of prior service cost (credit) 0 0 0 0
Amortization of net loss 469 142 939 284
Total net periodic pension cost $ 1,522 $ 1,548 $ 3,044 $ 3,096