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Pensions and other postretirement benefit plans - Schedule of Retirement Plan Costs (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2025
Jun. 30, 2024
Jun. 30, 2025
Jun. 30, 2024
Pension benefits        
Operating expenses:        
Service cost - benefits earned during the period $ 248 $ 289 $ 496 $ 578
Non-operating expenses:        
Interest cost on benefit obligations 20,837 20,276 41,224 40,591
Expected return on plan assets (23,444) (24,050) (46,304) (48,153)
Amortization of prior service cost (benefit) 18 17 35 34
Amortization of actuarial cost (benefit) 620 710 1,197 1,425
Total non-operating benefit (1,969) (3,047) (3,848) (6,103)
Total benefit for retirement plans (1,721) (2,758) (3,352) (5,525)
Postretirement benefits        
Operating expenses:        
Service cost - benefits earned during the period 8 8 16 17
Non-operating expenses:        
Interest cost on benefit obligations 526 530 1,052 1,060
Expected return on plan assets 0 0 0 0
Amortization of prior service cost (benefit) (142) (142) (284) (284)
Amortization of actuarial cost (benefit) (418) (478) (837) (956)
Total non-operating benefit (34) (90) (69) (180)
Total benefit for retirement plans $ (26) $ (82) $ (53) $ (163)