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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, 2024
Jun. 30, 2023
Jun. 30, 2024
Jun. 30, 2023
Non-operating expenses:        
Total non-operating (benefit) expense $ (3,137) $ (2,263) $ (6,283) $ (4,078)
Pension benefits        
Operating expenses:        
Service cost - benefits earned during the period 289 346 578 670
Non-operating expenses:        
Interest cost on benefit obligations 20,276 20,894 40,591 42,095
Expected return on plan assets (24,050) (23,814) (48,153) (47,482)
Amortization of prior service cost (benefit) 17 17 34 33
Amortization of actuarial cost (benefit) 710 544 1,425 1,084
Total non-operating (benefit) expense (3,047) (2,359) (6,103) (4,270)
Total (benefit) expense for retirement plans (2,758) (2,013) (5,525) (3,600)
Postretirement benefits        
Operating expenses:        
Service cost - benefits earned during the period 8 10 17 20
Non-operating expenses:        
Interest cost on benefit obligations 530 632 1,060 1,264
Expected return on plan assets 0 0 0 0
Amortization of prior service cost (benefit) (142) 0 (284) 0
Amortization of actuarial cost (benefit) (478) (536) (956) (1,072)
Total non-operating (benefit) expense (90) 96 (180) 192
Total (benefit) expense for retirement plans $ (82) $ 106 $ (163) $ 212