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Pension and postretirement benefits - Components of net periodic benefit cost (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2022
Mar. 31, 2021
Pension Plans    
Personnel costs:    
Service cost $ 0 $ 0
Other operating expenses    
Interest cost 4,800 3,998
Expected return on plan assets (8,847) (9,670)
Amortization of prior service cost/(credit) 0 0
Amortization of net loss 3,911 4,720
Total net periodic pension cost (136) (952)
OPEB Plan    
Personnel costs:    
Service cost 121 159
Other operating expenses    
Interest cost 983 893
Expected return on plan assets 0 0
Amortization of prior service cost/(credit) 0 0
Amortization of net loss 0 470
Total net periodic pension cost $ 1,104 $ 1,522