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Postretirement benefits (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Net periodic benefit income    
Interest cost $ 435 $ 496
Expected return on plan assets (2,099) (1,830)
Amortization of prior service credit (355) (355)
Amortization of net actuarial losses 334 568
Net periodic benefit income $ (1,685) $ (1,121)