Pension and postretirement benefit - Contributions to the benefit plans (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2023 USD ($)
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contribution	$ 114
Expected future employer contribution current year	228
OPEB Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contribution	3,258
Expected future employer contribution current year	$ 5,924