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Postretirement benefits (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Net periodic benefit income        
Interest cost $ 242 $ 478 $ 726 $ 1,434
Expected return on plan assets (1,875) (1,905) (5,623) (5,714)
Amortization of prior service credit (355) (355) (1,066) (1,066)
Amortization of net actuarial losses 407 575 1,221 1,725
Net periodic benefit income $ (1,581) $ (1,207) $ (4,742) $ (3,621)