Employee benefits - Accumulated benefit obligation less than plan assets (Details) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Project benefit obligation	$ 555,765	$ 599,829
Accumulated benefit obligation	555,765	599,829
Fair value of plan assets	588,998	623,233
OPEB Plan
Defined Benefit Plan Disclosure [Line Items]
Project benefit obligation	0	0
Accumulated benefit obligation	0	0
Fair value of plan assets	$ 0	$ 0