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Postretirement benefits (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2022
Sep. 30, 2021
Sep. 30, 2022
Sep. 30, 2021
Net periodic benefit income        
Interest cost $ 280 $ 242 $ 841 $ 726
Expected return on plan assets (1,866) (1,875) (5,596) (5,623)
Amortization of prior service credit (355) (355) (1,066) (1,066)
Amortization of net actuarial losses 225 407 674 1,221
Net periodic benefit income $ (1,716) $ (1,581) $ (5,147) $ (4,742)