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Employee benefits - Components of net periodic benefit cost (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Pension Plans      
Personnel costs:      
Service cost $ 0 $ 0 $ 0
Other operating expenses      
Interest cost 15,993 23,389 28,439
Expected return on plan assets (38,679) (38,104) (32,388)
Recognized net actuarial loss 18,876 20,880 23,508
Net periodic benefit (credit) cost (3,810) 6,165 19,559
Total net periodic pension cost (3,810) 6,165 19,559
OPEB Plan      
Personnel costs:      
Service cost 642 713 759
Other operating expenses      
Interest cost 3,573 4,913 5,955
Expected return on plan assets 0 0 0
Recognized net actuarial loss 1,873 567 0
Net periodic benefit (credit) cost 6,088 6,193 6,714
Total net periodic pension cost $ 6,088 $ 6,193 $ 6,714