Pension and postretirement benefits - Contributions to the benefit plans (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2021
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contribution	$ 172
Expected future employer contribution current year		$ 229
OPEB Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contribution	$ 4,640
Expected future employer contribution current year		$ 6,333