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Pension and postretirement benefits - Contributions to the benefit plans (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2022
Dec. 31, 2022
Pension Plans    
Defined Benefit Plans and Other Postretirement Benefit Plans    
Employer contribution $ 114  
Expected future employer contribution current year   $ 227
OPEB Plan    
Defined Benefit Plans and Other Postretirement Benefit Plans    
Employer contribution $ 3,305  
Expected future employer contribution current year   $ 5,971