Employee benefits - Contributions to the benefit plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contribution	$ 230	$ 230
Expected employer contributions to the benefit plans during the next year	227
OPEB Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contribution	6,899	$ 6,384
Expected employer contributions to the benefit plans during the next year	$ 5,428