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Pensions and other postretirement benefit plans - Schedule of Retirement Plan Costs (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2024
Sep. 30, 2023
Sep. 30, 2024
Sep. 30, 2023
Non-operating expenses:        
Total non-operating benefit $ (3,193) $ (2,929) $ (9,476) $ (7,007)
Pension benefits        
Operating expenses:        
Service cost - benefits earned during the period 288 335 866 1,005
Non-operating expenses:        
Interest cost on benefit obligations 20,501 21,248 61,092 63,343
Expected return on plan assets (24,356) (24,015) (72,509) (71,497)
Amortization of prior service cost (benefit) 18 18 52 51
Amortization of actuarial cost (benefit) 735 556 2,160 1,640
Total non-operating benefit (3,102) (2,193) (9,205) (6,463)
Total benefit for retirement plans (2,814) (1,858) (8,339) (5,458)
Postretirement benefits        
Operating expenses:        
Service cost - benefits earned during the period 9 10 26 30
Non-operating expenses:        
Interest cost on benefit obligations 530 486 1,590 1,750
Expected return on plan assets 0 0 0 0
Amortization of prior service cost (benefit) (143) (427) (427) (427)
Amortization of actuarial cost (benefit) (478) (795) (1,434) (1,867)
Total non-operating benefit (91) (736) (271) (544)
Total benefit for retirement plans $ (82) $ (726) $ (245) $ (514)