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Postretirement benefits (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2023
Jun. 30, 2022
Jun. 30, 2023
Jun. 30, 2022
Net periodic benefit income        
Interest cost $ 496 $ 280 $ 993 $ 561
Expected return on plan assets (1,830) (1,866) (3,660) (3,731)
Amortization of prior service credit (355) (355) (711) (711)
Amortization of net actuarial losses 568 225 1,137 450
Net periodic benefit income $ (1,121) $ (1,716) $ (2,241) $ (3,431)