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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, 2024
Jun. 30, 2023
Jun. 30, 2024
Jun. 30, 2023
Pension Plans        
Personnel costs:        
Service cost $ 0 $ 0 $ 0 $ 0
Other operating expenses        
Interest cost 7,558 7,887 15,117 15,774
Expected return on plan assets (8,594) (8,591) (17,188) (17,183)
Amortization of prior service cost/(credit) 0 0 0 0
Amortization of net loss 4,166 5,366 8,332 10,732
Total net periodic pension cost 3,130 4,662 6,261 9,323
OPEB Plan        
Personnel costs:        
Service cost 32 48 63 95
Other operating expenses        
Interest cost 1,421 1,520 2,843 3,041
Expected return on plan assets 0 0 0 0
Amortization of prior service cost/(credit) 0 0 0 0
Amortization of net loss (548) (553) (1,096) (1,106)
Total net periodic pension cost $ 905 $ 1,015 $ 1,810 $ 2,030