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Pensions and other postretirement benefit plans - Schedule of Retirement Plan Costs (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Pension benefits    
Operating expenses:    
Service cost - benefits earned during the period $ 248 $ 289
Non-operating expenses:    
Interest cost on benefit obligations 20,387 20,315
Expected return on plan assets (22,860) (24,103)
Amortization of prior service cost (benefit) 17 17
Amortization of actuarial cost (benefit) 577 715
Total non-operating benefit (1,879) (3,056)
Total benefit for retirement plans (1,631) (2,767)
Postretirement benefits    
Operating expenses:    
Service cost - benefits earned during the period 8 9
Non-operating expenses:    
Interest cost on benefit obligations 526 530
Expected return on plan assets 0 0
Amortization of prior service cost (benefit) (142) (142)
Amortization of actuarial cost (benefit) (419) (478)
Total non-operating benefit (35) (90)
Total benefit for retirement plans $ (27) $ (81)