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Pension and postretirement benefits - Components of net periodic benefit cost (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Pension Plans        
Personnel costs:        
Service cost $ 0 $ 0 $ 0 $ 0
Other operating expenses        
Interest cost 5,847 7,109 17,541 21,329
Expected return on plan assets (9,526) (8,096) (28,577) (24,288)
Amortization of prior service cost (credit) 0 0 0 0
Amortization of net loss 5,220 5,877 15,660 17,629
Total net periodic pension cost 1,541 4,890 4,624 14,670
OPEB Plan        
Personnel costs:        
Service cost 178 190 535 570
Other operating expenses        
Interest cost 1,228 1,489 3,684 4,466
Expected return on plan assets 0 0 0 0
Amortization of prior service cost (credit) 0 0 0 0
Amortization of net loss 142 0 426 0
Total net periodic pension cost $ 1,548 $ 1,679 $ 4,645 $ 5,036