v3.25.1
Pension and postretirement benefits - Components of net periodic benefit cost (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Pension Plans    
Personnel costs:    
Service cost $ 0 $ 0
Other operating expenses    
Interest cost 7,411 7,558
Expected return on plan assets (8,069) (8,594)
Amortization of prior service cost/(credit) 0 0
Amortization of net loss 3,450 4,166
Total net periodic pension cost 2,792 3,130
OPEB Plan    
Personnel costs:    
Service cost 15 32
Other operating expenses    
Interest cost 1,291 1,421
Expected return on plan assets 0 0
Amortization of prior service cost/(credit) 0 0
Amortization of net loss (1,177) (548)
Total net periodic pension cost $ 129 $ 905