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Postretirement benefits (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2022
Jun. 30, 2021
Jun. 30, 2022
Jun. 30, 2021
Net periodic benefit income        
Interest cost $ 280 $ 242 $ 561 $ 484
Expected return on plan assets (1,866) (1,875) (3,731) (3,748)
Amortization of prior service credit (355) (355) (711) (711)
Amortization of net actuarial losses 225 407 450 814
Net periodic benefit income $ (1,716) $ (1,581) $ (3,431) $ (3,161)